feat(cardav): create cardav router with GET /accounts

Add CardDAV API router with GET /accounts endpoint. Returns all
CardDAV accounts from database via cardav-sync service. Added test
infrastructure with _setTestDatabase helper for isolated API testing.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

server/routes/cardav.js

@@ -0,0 +1,33 @@
+/**
+ * Modul: CardDAV Management
+ * Zweck: REST-API-Routen für CardDAV Account Management, Addressbook Discovery, Sync
+ * Abhängigkeiten: express, server/db.js, server/services/cardav-sync.js
+ */
+
+import { createLogger } from '../logger.js';
+import express from 'express';
+import * as db from '../db.js';
+import * as CardDAVSync from '../services/cardav-sync.js';
+import { str, collectErrors, MAX_TITLE } from '../middleware/validate.js';
+
+const log = createLogger('CardDAV');
+const MAX_URL = 500;
+
+const router = express.Router();
+
+/**
+ * GET /api/v1/contacts/cardav/accounts
+ * Liste aller CardDAV Accounts.
+ * Response: { data: Account[] }
+ */
+router.get('/accounts', async (req, res) => {
+  try {
+    const accounts = await CardDAVSync.getAllAccounts();
+    res.json({ data: accounts });
+  } catch (err) {
+    log.error('Error fetching accounts:', err);
+    res.status(500).json({ error: err.message || 'Interner Fehler', code: 500 });
+  }
+});
+
+export default router;