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/package-lock.json

@@ -27,6 +27,7 @@
         "jsonwebtoken": "^9.0.2",
         "jwks-rsa": "^3.1.0",
         "multer": "^2.0.1",
+        "nodemailer": "^7.0.13",
         "passport": "^0.7.0",
         "passport-jwt": "^4.0.1",
         "reflect-metadata": "^0.2.2",
@@ -44,6 +45,7 @@
         "@types/jsonwebtoken": "^9.0.7",
         "@types/multer": "^1.4.12",
         "@types/node": "^20.17.9",
+        "@types/nodemailer": "^6.4.24",
         "@types/passport": "^1.0.17",
         "@types/passport-jwt": "^4.0.1",
         "@types/supertest": "^6.0.2",
@@ -2985,6 +2987,16 @@
         "undici-types": "~6.21.0"
       }
     },
+    "node_modules/@types/nodemailer": {
+      "version": "6.4.24",
+      "resolved": "https://registry.npmjs.org/@types/nodemailer/-/nodemailer-6.4.24.tgz",
+      "integrity": "sha512-Ww4u0rT9wQNXh4JiQaIwx3QWdcOFXzOjQA2zc+jtFYNmQiT4mIUqcDin51bDFdkzKubFnQCZNK7FIHlPKQ/q9w==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/passport": {
       "version": "1.0.17",
       "resolved": "https://registry.npmjs.org/@types/passport/-/passport-1.0.17.tgz",
@@ -8374,6 +8386,15 @@
         "node": ">=18"
       }
     },
+    "node_modules/nodemailer": {
+      "version": "7.0.13",
+      "resolved": "https://registry.npmjs.org/nodemailer/-/nodemailer-7.0.13.tgz",
+      "integrity": "sha512-PNDFSJdP+KFgdsG3ZzMXCgquO7I6McjY2vlqILjtJd0hy8wEvtugS9xKRF2NWlPNGxvLCXlTNIae4serI7dinw==",
+      "license": "MIT-0",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/normalize-path": {
       "version": "3.0.0",
       "resolved": "https://registry.npmjs.org/normalize-path/-/normalize-path-3.0.0.tgz",