feat(data-safety): scheduled DB backups + audit trail + verified actor attribution

- db-backup sidecar: pg_dump -Fc of procurement+casdoor every
  BACKUP_INTERVAL_SECONDS into BACKUP_DIR, retention via BACKUP_KEEP_DAYS
- audit_log table (migration.sql + runtime ensure_audit_table) with
  model + transactional helper (utils/audit.record) — wired into user
  create/role-change and inventory transaction creation
- get_actor: quote_events.actor now stores the VERIFIED user's display
  name instead of the raw bearer token

backend/app/utils/audit.py

@@ -0,0 +1,41 @@
+"""Audit helper — one row per sensitive mutation, written in the SAME
+transaction as the change (rolled back together with it).
+
+    await record(db, actor=ctx.display_name, action="user.role_changed",
+                 entity="user", entity_id=str(user.id),
+                 details={"from": old_role, "to": user.role})
+
+Never raises: an audit failure must not fail the business operation.
+"""
+import logging
+from typing import Any
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.audit import AuditLog
+
+logger = logging.getLogger("procurement.audit")
+
+
+async def record(
+    db: AsyncSession,
+    *,
+    actor: str | None,
+    action: str,
+    entity: str,
+    entity_id: str | None = None,
+    details: dict[str, Any] | None = None,
+) -> None:
+    try:
+        db.add(
+            AuditLog(
+                actor=actor,
+                action=action,
+                entity=entity,
+                entity_id=str(entity_id) if entity_id else None,
+                details=details or {},
+            )
+        )
+        await db.flush()
+    except Exception:  # noqa: BLE001 — audit must never break the caller
+        logger.exception("failed to write audit_log row (%s/%s)", action, entity)