feat(PROJ-8): Automatischer IMAP-Sync (Cron-Scheduler)

Backend:
- internal/imap/store.go: 7 neue Felder (sync_interval_min, last_sync_at,
  last_sync_count, last_uid, sync_running, sync_status, sync_error_msg)
  DB-Migration via ALTER TABLE ADD COLUMN IF NOT EXISTS
  Neue Methoden: ListAll, UpdateSyncInterval, SetSyncRunning, UpdateSyncResult
- internal/imap/scheduler.go: Scheduler mit time.Ticker (1 min),
  inkrementeller Sync via UID SEARCH UID <lastUID+1>:*,
  exponential backoff (3 Versuche: 1s / 60s / 300s),
  sync_running-Flag verhindert parallele Syncs
- internal/api/server.go: POST /api/imap/{id}/sync (manueller Trigger),
  PATCH /api/imap/{id} (sync_interval_min setzen, 0 oder 5-1440 min)
- cmd/archivmail/main.go: Scheduler gestartet + via SetImap verdrahtet

Frontend:
- src/lib/api.ts: 6 neue ImapAccount-Felder, triggerImapSync, updateImapInterval
- src/app/imap/page.tsx: Intervall-Dropdown, "Sync jetzt"-Button,
  Letzter-Sync-Anzeige mit Status-Badge, Polling auch bei sync_running

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/api.ts

@@ -314,6 +314,13 @@ export interface ImapAccount {
   progress_current: number;
   progress_total: number;
   created_at: string;
+  // PROJ-8: Auto-sync fields
+  sync_interval_min: number;
+  last_sync_at?: string;
+  last_sync_count: number;
+  sync_running: boolean;
+  sync_status: string;
+  sync_error_msg: string;
 }
 
 export interface ImapTestResult {
@@ -366,6 +373,17 @@ export async function getImapProgress(id: number): Promise<ImapAccount> {
   return request<ImapAccount>(`/api/imap/${id}/progress`);
 }
 
+export async function triggerImapSync(id: number): Promise<ImapAccount> {
+  return request<ImapAccount>(`/api/imap/${id}/sync`, { method: "POST" });
+}
+
+export async function updateImapInterval(id: number, intervalMin: number): Promise<ImapAccount> {
+  return request<ImapAccount>(`/api/imap/${id}`, {
+    method: "PATCH",
+    body: JSON.stringify({ sync_interval_min: intervalMin }),
+  });
+}
+
 // ── System Stats ──────────────────────────────────────────────────────────
 
 export interface SystemStatsCPU {