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/i18n/locales/en/audit.json

@@ -0,0 +1,18 @@
+{
+  "title": "Audit log",
+  "intro": "An append-only record of admin actions and scheduler run summaries — who did what, and when. Logged at action granularity, never per item.",
+  "empty": "No audit entries yet.",
+  "system": "System",
+  "clear": "Clear log",
+  "clearTitle": "Clear the audit log?",
+  "clearConfirm": "Permanently delete all audit entries? This can't be undone. (One entry recording that you cleared it is kept.)",
+  "cleared": "Cleared {{count}} audit entries",
+  "clearFailed": "Couldn't clear the audit log.",
+  "truncated": "Showing the {{returned}} most recent of {{total}} entries.",
+  "cols": {
+    "when": "When",
+    "actor": "Who",
+    "action": "Action",
+    "details": "Details"
+  }
+}