feat: add gateway data collection service + fix model FK definitions
- Created src/backend/app/services/monitoring/ package:
- gateway_collector.py: Background asyncio task that polls gateway RPC endpoints
(usage.cost, usage.status, cron.list, sessions.list/preview, health, status)
and stores results in monitoring models using upsert pattern
- models.py: Pydantic schemas for parsing gateway RPC responses
- __init__.py: Package init, exports GatewayCollectorService
- Added collector startup/shutdown in main.py lifespan:
- Launches collector as background task when gateways exist
- Clean shutdown on app termination
- Fixed model FK definitions in monitoring.py and alert_rules.py:
- Replaced Column(UUID, ForeignKey(...)) with Field(foreign_key=...)
to match codebase pattern (UUID is Python class, not SQLAlchemy type)
- Added missing gateway_id field to AlertRule model
- Removed OpenClawDBService inheritance from GatewayCollectorService
(uses session factory pattern instead of injected session)
- Cleaned up duplicate/conflicting imports
- Configurable collection intervals via env vars:
COLLECTION_INTERVAL_COST (300s), COLLECTION_INTERVAL_CRON (60s),
COLLECTION_INTERVAL_SESSION (30s), COLLECTION_INTERVAL_HEALTH (60s)
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