feat(audit): admin audit log (Notification P3)

Append-only who/what/when trail for admin actions + scheduler run summaries,
generalizing QuotaEvent. Logged at ACTION / run-summary granularity — never
per-row (a scheduler run touching thousands of videos is ONE row).

Backend:
- migration 0054_audit_log + AuditLog model (actor_id FK SET NULL, action,
  target_type/id, summary, before/after JSON, created_at).
- app/audit.py: AuditAction constants (single source of truth, i18n'd on the
  client) + record() (adds to the caller's txn) + gc(retention_days).
- producers wired: role change / suspend / delete / invite approve-deny-add /
  demo add-remove-reset / discovery purge (admin.py); config set/reset — secret
  VALUES never logged, key only (config.py); scheduler interval + maintenance
  tune (scheduler routes); sync pause/resume (sync.py); and the scheduler _job()
  choke point writes ONE run-summary row per run — manual runs + errors always,
  automatic runs only when they changed something (no no-op-poll spam).
- retention: audit_gc scheduler job prunes rows older than audit_retention_days
  (sysconfig, default 180; 0 = keep forever).
- admin API routes/audit.py (/api/admin/audit): recent-window list (actor emails
  resolved, System for background) + clear (leaves one tamper-evidence row).

Frontend:
- new admin-only "Audit log" nav page (ScrollText) using the shared DataTable
  (first admin page to reuse it) — sort/filter/paginate/persist, action select
  filter, actor text filter, before→after detail, Clear-all with confirm.
- AuditLog.tsx + auditColumns.tsx + api.adminAudit/clearAudit + AuditRow type.
- trilingual audit + auditActions namespaces (HU/EN/DE) + nav + config-group +
  scheduler job labels; Audit config group (retention days).

frontend/src/App.tsx

@@ -73,6 +73,7 @@ const Stats = lazy(() => import("./components/Stats"));
 const Scheduler = lazy(() => import("./components/Scheduler"));
 const ConfigPanel = lazy(() => import("./components/ConfigPanel"));
 const AdminUsers = lazy(() => import("./components/AdminUsers"));
+const AuditLog = lazy(() => import("./components/AuditLog"));
 const SettingsPanel = lazy(() => import("./components/SettingsPanel"));
 const NotificationsPanel = lazy(() => import("./components/NotificationsPanel"));
 const Messages = lazy(() => import("./components/Messages"));
@@ -828,6 +829,8 @@ export default function App() {
             <ConfigPanel />
           ) : page === "users" && meQuery.data!.role === "admin" ? (
             <AdminUsers me={meQuery.data!} />
+          ) : page === "audit" && meQuery.data!.role === "admin" ? (
+            <AuditLog />
           ) : page === "playlists" ? (
             <Playlists canWrite={meQuery.data!.can_write} />
           ) : page === "notifications" ? (