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).

backend/app/routes/sync.py

@@ -2,7 +2,8 @@ from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import and_, case, func, select, update
 from sqlalchemy.orm import Session
 
-from app import quota, state
+from app import audit, quota, state
+from app.audit import AuditAction
 from app.auth import admin_user, current_user, has_read_scope, require_human
 from app.db import get_db
 from app.models import Channel, Subscription, User, Video
@@ -188,12 +189,16 @@ def deep_all(
 
 
 @router.post("/pause")
-def pause_sync(_: User = Depends(admin_user), db: Session = Depends(get_db)) -> dict:
+def pause_sync(admin: User = Depends(admin_user), db: Session = Depends(get_db)) -> dict:
     state.set_sync_paused(db, True)
+    audit.record(db, admin.id, AuditAction.SYNC_PAUSE, summary="Background sync paused")
+    db.commit()
     return {"paused": True}
 
 
 @router.post("/resume")
-def resume_sync(_: User = Depends(admin_user), db: Session = Depends(get_db)) -> dict:
+def resume_sync(admin: User = Depends(admin_user), db: Session = Depends(get_db)) -> dict:
     state.set_sync_paused(db, False)
+    audit.record(db, admin.id, AuditAction.SYNC_RESUME, summary="Background sync resumed")
+    db.commit()
     return {"paused": False}