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>

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) {