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/.env.example

@@ -17,6 +17,20 @@ TEAM_JWT_SECRET="change-me-too"
 
 PORT=3000
 
+# Public base URL of the app, used to build links in outgoing emails.
+APP_BASE_URL="http://localhost:3000"
+
+# Email: defaults to "log" (writes what it would send to the log, no
+# delivery). Set MAIL_PROVIDER=smtp plus the SMTP_* vars + MAIL_FROM to
+# actually send Gemeinde-Teamer invite emails.
+MAIL_PROVIDER="log"
+MAIL_FROM="KC-App <no-reply@example.org>"
+SMTP_HOST="smtp.example.org"
+SMTP_PORT=587
+SMTP_SECURE="false"
+SMTP_USER=""
+SMTP_PASS=""
+
 # File storage: defaults to Nextcloud via WebDAV; set STORAGE_PROVIDER=s3 to
 # use an S3-compatible bucket instead (see S3_* vars below).
 STORAGE_PROVIDER="webdav"