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/models/audit.py

@@ -0,0 +1,19 @@
+"""Audit log model — general activity trail (v2.7)."""
+import uuid
+
+from sqlalchemy import Column, DateTime, String, Text, func
+from sqlalchemy.dialects.postgresql import JSONB, UUID
+
+from app.db.base import Base
+
+
+class AuditLog(Base):
+    __tablename__ = "audit_log"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    occurred_at = Column(DateTime(timezone=True), nullable=False, server_default=func.now())
+    actor = Column(Text, nullable=True)  # display name of the authenticated user
+    action = Column(String(64), nullable=False)  # e.g. "user.role_changed"
+    entity = Column(String(64), nullable=False)  # e.g. "user", "inventory_tx"
+    entity_id = Column(Text, nullable=True)
+    details = Column(JSONB, nullable=False, server_default="{}")