Add subscription system for graph change notifications

[ravisuhag]

Context

AI agents and automated systems monitoring data pipelines need reactive notifications, not just pull-based queries. Compass should support subscriptions that notify consumers when specific entities or graph regions change.

Scope

• Subscribe to changes on specific entities, entity types, namespaces, or relationship patterns
• Notification triggers: schema change, ownership transfer, lineage modification, quality degradation, new entity
• Delivery channels: webhook callbacks, change feed filtered subscription
• Subscription management API: create, list, update, delete subscriptions

Examples

• "Notify me when the schema of orders table changes"
• "Alert when a new entity appears in the payments namespace"
• "Watch for lineage changes affecting the revenue dashboard"

Dependencies

• Change feed API (Add change feed API for streaming graph mutations to consumers #238)

Why

Polling is inefficient for monitoring. Subscriptions let AI agents and automation react to changes as they happen, keeping cached context fresh and triggering workflows when something important changes.

References

• Compass Roadmap — Active Knowledge