feat(caldav): add caldav-sync service base structure with helpers

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/services/caldav-sync.js

@@ -0,0 +1,62 @@
+/**
+ * Modul: Generic CalDAV Sync
+ * Zweck: Multi-Account CalDAV synchronization with calendar selection
+ * Abhängigkeiten: tsdav, server/db.js, server/services/ics-parser.js
+ */
+
+import { createLogger } from '../logger.js';
+const log = createLogger('CalDAV');
+
+import * as db from '../db.js';
+
+// Reused functions from apple-calendar.js
+import {
+  parseICS,
+  buildICS,
+  escapeICS,
+  unescapeICS,
+  formatICSDate,
+  tzLocalToUTC,
+  applyDuration
+} from './ics-parser.js';
+
+// --------------------------------------------------------
+// Helper Functions
+// --------------------------------------------------------
+
+function normalizeCalColor(c) {
+  if (!c) return null;
+  if (/^#[0-9a-fA-F]{8}$/.test(c)) return c.slice(0, 7); // strip alpha
+  if (/^#[0-9a-fA-F]{6}$/.test(c)) return c;
+  return null;
+}
+
+function upsertExternalCalendar(source, externalId, name, color) {
+  const row = db.get().prepare(`
+    INSERT INTO external_calendars (source, external_id, name, color)
+    VALUES (?, ?, ?, ?)
+    ON CONFLICT(source, external_id) DO UPDATE SET
+      name  = excluded.name,
+      color = excluded.color
+    RETURNING id
+  `).get(source, externalId, name, color);
+  return row.id;
+}
+
+// --------------------------------------------------------
+// Credentials Helpers
+// --------------------------------------------------------
+
+function getAccountById(accountId) {
+  return db.get().prepare('SELECT * FROM caldav_accounts WHERE id = ?').get(accountId);
+}
+
+function getAllAccounts() {
+  return db.get().prepare('SELECT * FROM caldav_accounts').all();
+}
+
+// --------------------------------------------------------
+// Export placeholder (will be filled in next tasks)
+// --------------------------------------------------------
+
+export { };