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/db/init_db.py

@@ -392,3 +392,27 @@ async def ensure_v26(engine: AsyncEngine) -> None:
             AFTER INSERT OR UPDATE OF status ON item_decisions
             FOR EACH ROW EXECUTE FUNCTION fn_sync_last_quoted_price()
     """)
+
+
+async def ensure_audit_table(engine: AsyncEngine) -> None:
+    """v2.7: general activity trail (audit_log). Idempotent; safe on any volume."""
+    ddl = """
+    CREATE TABLE IF NOT EXISTS audit_log (
+        id          UUID        NOT NULL DEFAULT gen_random_uuid() PRIMARY KEY,
+        occurred_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+        actor       TEXT,
+        action      TEXT        NOT NULL,
+        entity      TEXT        NOT NULL,
+        entity_id   TEXT,
+        details     JSONB       NOT NULL DEFAULT '{}'
+    )
+    """
+    indexes = (
+        "CREATE INDEX IF NOT EXISTS idx_audit_log_entity ON audit_log(entity, entity_id)",
+        "CREATE INDEX IF NOT EXISTS idx_audit_log_occurred ON audit_log(occurred_at DESC)",
+    )
+    async with engine.begin() as conn:
+        await conn.execute(text(ddl))
+        for stmt in indexes:
+            await conn.execute(text(stmt))
+    logger.info("audit_log table ensured")