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.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 () => {