feat(backend): mail module + send personal Gemeinde-Teamer invites

New global mail/ module mirroring the files/storage/ provider pattern:
- MailProvider abstraction; default LogMailProvider only logs (no delivery),
  MAIL_PROVIDER=smtp switches to a nodemailer SMTP transport (SMTP_*,
  MAIL_FROM).
- MailService.sendTeamerInvite() composes the invite email with a link
  built from APP_BASE_URL.

TeamerService.createInvite() now mails personal invites (those with an
email) best-effort and returns `emailSent`; group links are unchanged.
Delivery failures are logged and swallowed, never blocking invite creation.

New env: APP_BASE_URL, MAIL_PROVIDER, MAIL_FROM, SMTP_HOST/PORT/SECURE/
USER/PASS. Tests: teamer spec covers mail-on-personal-invite,
no-mail-on-group-link, and transport-drop; npm test green at 56. Docs
updated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/app.module.ts

@@ -3,6 +3,7 @@ import { ConfigModule } from '@nestjs/config';
 import { ServeStaticModule } from '@nestjs/serve-static';
 import { join } from 'path';
 import { PrismaModule } from './prisma/prisma.module';
+import { MailModule } from './mail/mail.module';
 import { AuthModule } from './auth/auth.module';
 import { KcModule } from './kc/kc.module';
 import { GemeindeModule } from './gemeinde/gemeinde.module';
@@ -23,6 +24,7 @@ import { SyncModule } from './sync/sync.module';
       exclude: ['/api*'],
     }),
     PrismaModule,
+    MailModule,
     SyncModule,
     AuthModule,
     KcModule,

backend/src/mail/log-mail.provider.ts

@@ -0,0 +1,15 @@
+import { Logger } from '@nestjs/common';
+import { MailMessage, MailProvider } from './mail-provider';
+
+/// Default provider: doesn't send anything, just logs that it would have.
+/// Keeps the invite flow working before SMTP is configured.
+export class LogMailProvider implements MailProvider {
+  private readonly logger = new Logger('MailProvider');
+
+  async send(message: MailMessage): Promise<boolean> {
+    this.logger.log(
+      `[log-only] would send "${message.subject}" to ${message.to}: ${message.text}`,
+    );
+    return false;
+  }
+}

backend/src/mail/mail-provider.ts

@@ -0,0 +1,18 @@
+/// Abstraction over the outbound email backend. Default is a no-send provider
+/// that only logs (fine for dev and for deployments that don't do email yet);
+/// MAIL_PROVIDER=smtp switches to a real SMTP transport.
+export interface MailMessage {
+  to: string;
+  subject: string;
+  text: string;
+  html?: string;
+}
+
+export interface MailProvider {
+  /// Resolves true if the message was handed off to the transport, false if
+  /// it was dropped (e.g. the log provider). Never throws for delivery
+  /// problems — callers treat email as best-effort.
+  send(message: MailMessage): Promise<boolean>;
+}
+
+export const MAIL_PROVIDER = Symbol('MAIL_PROVIDER');

backend/src/mail/mail.module.ts

@@ -0,0 +1,25 @@
+import { Global, Module } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+import { MAIL_PROVIDER } from './mail-provider';
+import { LogMailProvider } from './log-mail.provider';
+import { SmtpMailProvider } from './smtp-mail.provider';
+import { MailService } from './mail.service';
+
+/// Global so any feature module can inject MailService. Provider defaults to
+/// log-only; MAIL_PROVIDER=smtp switches to a real SMTP transport.
+@Global()
+@Module({
+  providers: [
+    MailService,
+    {
+      provide: MAIL_PROVIDER,
+      inject: [ConfigService],
+      useFactory: (config: ConfigService) =>
+        config.get<string>('MAIL_PROVIDER') === 'smtp'
+          ? new SmtpMailProvider(config)
+          : new LogMailProvider(),
+    },
+  ],
+  exports: [MailService],
+})
+export class MailModule {}

backend/src/mail/mail.service.ts

@@ -0,0 +1,43 @@
+import { Inject, Injectable } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+import { MAIL_PROVIDER, MailProvider } from './mail-provider';
+
+@Injectable()
+export class MailService {
+  private readonly appBaseUrl: string;
+
+  constructor(
+    @Inject(MAIL_PROVIDER) private readonly provider: MailProvider,
+    config: ConfigService,
+  ) {
+    this.appBaseUrl = (config.get<string>('APP_BASE_URL') ?? 'http://localhost:3000').replace(
+      /\/$/,
+      '',
+    );
+  }
+
+  /// Sends a personal Gemeinde-Teamer invite. Returns whether it was handed
+  /// to the transport (false for the log-only provider or on failure).
+  sendTeamerInvite(opts: {
+    to: string;
+    kcName: string;
+    gemeindeName: string;
+    token: string;
+    expiresAt: Date | null;
+  }): Promise<boolean> {
+    const link = `${this.appBaseUrl}/?teamerInviteToken=${encodeURIComponent(opts.token)}`;
+    const expiry = opts.expiresAt
+      ? `\n\nDer Link gilt bis ${opts.expiresAt.toISOString()}.`
+      : '';
+    return this.provider.send({
+      to: opts.to,
+      subject: `Einladung als Teamer:in – ${opts.gemeindeName} (${opts.kcName})`,
+      text:
+        `Hallo,\n\ndu wurdest als Teamer:in für die Gemeinde "${opts.gemeindeName}" ` +
+        `beim ${opts.kcName} eingeladen.\n\n` +
+        `Konto anlegen: ${link}\n\n` +
+        `Falls der Link nicht funktioniert, nutze diesen Einladungscode: ${opts.token}` +
+        `${expiry}\n`,
+    });
+  }
+}

backend/src/mail/smtp-mail.provider.ts

@@ -0,0 +1,45 @@
+import { Logger } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+import * as nodemailer from 'nodemailer';
+import { MailMessage, MailProvider } from './mail-provider';
+
+/// SMTP transport (MAIL_PROVIDER=smtp). Delivery failures are logged and
+/// swallowed — callers treat email as best-effort.
+export class SmtpMailProvider implements MailProvider {
+  private readonly logger = new Logger('MailProvider');
+  private readonly from: string;
+  private readonly transport: nodemailer.Transporter;
+
+  constructor(config: ConfigService) {
+    this.from = config.getOrThrow<string>('MAIL_FROM');
+    this.transport = nodemailer.createTransport({
+      host: config.getOrThrow<string>('SMTP_HOST'),
+      port: Number(config.get<string>('SMTP_PORT') ?? 587),
+      secure: config.get<string>('SMTP_SECURE') === 'true',
+      auth: config.get<string>('SMTP_USER')
+        ? {
+            user: config.getOrThrow<string>('SMTP_USER'),
+            pass: config.getOrThrow<string>('SMTP_PASS'),
+          }
+        : undefined,
+    });
+  }
+
+  async send(message: MailMessage): Promise<boolean> {
+    try {
+      await this.transport.sendMail({
+        from: this.from,
+        to: message.to,
+        subject: message.subject,
+        text: message.text,
+        html: message.html,
+      });
+      return true;
+    } catch (err) {
+      this.logger.error(
+        `Failed to send "${message.subject}" to ${message.to}: ${(err as Error).message}`,
+      );
+      return false;
+    }
+  }
+}

backend/src/teamer/teamer.service.spec.ts

@@ -31,6 +31,7 @@ function makeService(opts: { gemeinde?: typeof GEMEINDE | null; existingEmails?:
 
   const prisma = {
     gemeinde: { findUnique: jest.fn().mockResolvedValue(gemeinde) },
+    kc: { findUnique: jest.fn().mockResolvedValue({ name: 'KC 2026' }) },
     user: {
       findUnique: jest.fn(({ where }: { where: { email: string } }) =>
         Promise.resolve(emails.has(where.email) ? { id: 'dup', email: where.email } : null),
@@ -56,8 +57,9 @@ function makeService(opts: { gemeinde?: typeof GEMEINDE | null; existingEmails?:
     },
   };
   const sync = { capture: jest.fn().mockResolvedValue(undefined) };
-  const service = new TeamerService(prisma as never, sync as never);
-  return { service, prisma, sync, created };
+  const mail = { sendTeamerInvite: jest.fn().mockResolvedValue(true) };
+  const service = new TeamerService(prisma as never, sync as never, mail as never);
+  return { service, prisma, sync, mail, created };
 }
 
 describe('TeamerService scope check', () => {
@@ -120,20 +122,34 @@ describe('TeamerService.createTeamer', () => {
 });
 
 describe('TeamerService.createInvite', () => {
-  it('defaults a group link to unlimited uses and no expiry', async () => {
-    const { service } = makeService();
+  it('defaults a group link to unlimited uses, no expiry, and sends no email', async () => {
+    const { service, mail } = makeService();
     const inv = await service.createInvite(LT, 'gem-1', {});
     expect(inv.email).toBeNull();
     expect(inv.maxUses).toBeNull();
     expect(inv.expiresAt).toBeNull();
     expect(inv.token).toEqual(expect.any(String));
+    expect(inv.emailSent).toBe(false);
+    expect(mail.sendTeamerInvite).not.toHaveBeenCalled();
   });
 
-  it('defaults a personal invite to a single use and lowercases the email', async () => {
-    const { service } = makeService();
+  it('defaults a personal invite to a single use, lowercases the email, and mails it', async () => {
+    const { service, mail } = makeService();
     const inv = await service.createInvite(LT, 'gem-1', { email: 'New@Example.org' });
     expect(inv.email).toBe('new@example.org');
     expect(inv.maxUses).toBe(1);
+    expect(inv.emailSent).toBe(true);
+    expect(mail.sendTeamerInvite).toHaveBeenCalledWith(
+      expect.objectContaining({ to: 'new@example.org', gemeindeName: 'Nord', kcName: 'KC 2026' }),
+    );
+  });
+
+  it('still returns the invite when the mail transport drops it', async () => {
+    const { service, mail } = makeService();
+    mail.sendTeamerInvite.mockResolvedValueOnce(false);
+    const inv = await service.createInvite(LT, 'gem-1', { email: 'x@example.org' });
+    expect(inv.emailSent).toBe(false);
+    expect(inv.token).toEqual(expect.any(String));
   });
 
   it('turns expiresInHours into a concrete expiry', async () => {

backend/src/teamer/teamer.service.ts

@@ -9,6 +9,7 @@ import { Role, SyncOperation } from '@prisma/client';
 import * as bcrypt from 'bcryptjs';
 import { PrismaClient } from '../prisma/prisma.module';
 import { SyncService } from '../sync/sync.service';
+import { MailService } from '../mail/mail.service';
 import { AuthenticatedUser } from '../auth/authenticated-request';
 import { CreateTeamerInviteDto } from './dto/create-teamer-invite.dto';
 
@@ -30,6 +31,7 @@ export class TeamerService {
   constructor(
     private readonly prisma: PrismaClient,
     private readonly sync: SyncService,
+    private readonly mail: MailService,
   ) {}
 
   async createTeamer(
@@ -118,7 +120,24 @@ export class TeamerService {
       },
     });
     await this.sync.capture('TeamerInvite', SyncOperation.CREATE, invite.id, invite);
-    return invite;
+
+    // Personal invites go out by email (best-effort); group links are shared
+    // by the Verantwortliche/r directly.
+    let emailSent = false;
+    if (email) {
+      const kc = await this.prisma.kc.findUnique({
+        where: { id: gemeinde.kcId },
+        select: { name: true },
+      });
+      emailSent = await this.mail.sendTeamerInvite({
+        to: email,
+        kcName: kc?.name ?? '',
+        gemeindeName: gemeinde.name,
+        token: invite.token,
+        expiresAt: invite.expiresAt,
+      });
+    }
+    return { ...invite, emailSent };
   }
 
   async listInvites(caller: AuthenticatedUser, gemeindeId: string) {