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/alembic/versions/0054_audit_log.py

@@ -0,0 +1,51 @@
+"""audit_log: append-only admin/scheduler audit trail (who/what/when/before->after)
+
+Revision ID: 0054_audit_log
+Revises: 0053_user_session_epoch
+Create Date: 2026-07-12
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0054_audit_log"
+down_revision: Union[str, None] = "0053_user_session_epoch"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "audit_log",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        # SET NULL on user delete: keep the trail (it just becomes system-attributed).
+        sa.Column(
+            "actor_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+        sa.Column("action", sa.String(length=48), nullable=False),
+        sa.Column("target_type", sa.String(length=32), nullable=True),
+        sa.Column("target_id", sa.String(length=128), nullable=True),
+        sa.Column("summary", sa.String(length=255), nullable=True),
+        sa.Column("before", sa.JSON(), nullable=True),
+        sa.Column("after", sa.JSON(), nullable=True),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+    )
+    op.create_index("ix_audit_log_actor_id", "audit_log", ["actor_id"])
+    op.create_index("ix_audit_log_action", "audit_log", ["action"])
+    op.create_index("ix_audit_log_created_at", "audit_log", ["created_at"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_audit_log_created_at", table_name="audit_log")
+    op.drop_index("ix_audit_log_action", table_name="audit_log")
+    op.drop_index("ix_audit_log_actor_id", table_name="audit_log")
+    op.drop_table("audit_log")