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/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 {}