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/db.js

@@ -1354,6 +1354,14 @@ function transaction(fn) {
   return get().transaction(fn)();
 }
 
+/**
+ * ONLY FOR TESTING: Override the internal db instance
+ * @param {import('better-sqlite3').Database} testDb
+ */
+function _setTestDatabase(testDb) {
+  db = testDb;
+}
+
 init();   // auto-initialise when module is first imported
 
-export { init, get, transaction, currentVersion, getPath, backupToFile, restoreFromFile, MIGRATIONS };
+export { init, get, transaction, currentVersion, getPath, backupToFile, restoreFromFile, MIGRATIONS, _setTestDatabase };