feat(invalidation): replace Postgres LISTEN/NOTIFY with GCP Pub/Sub

[steventohme]

Summary

• Removes the long-lived Postgres connection held per replica for cache invalidation (LISTEN router_installation_invalidate) — showed up as long-running transactions and burned connection pool slots unnecessarily
• Adds internal/pubsub/invalidation.go with InvalidationNotifier (publishes installationID) and InvalidationListener (subscribes via Receive) backed by cloud.google.com/go/pubsub/v2
• Deletes internal/postgres/invalidation.go (PgxInvalidationNotifier + InvalidationListener using WaitForNotification)
• Wires up in main.go from three new required env vars: PUBSUB_PROJECT_ID, PUBSUB_TOPIC_ROUTER_INVALIDATION, PUBSUB_SUBSCRIPTION_ROUTER_INVALIDATION
• The 5-min cache TTL remains as the safety net for missed invalidations

New env vars required

Var	Example
PUBSUB_PROJECT_ID	workweave-prod-01
PUBSUB_TOPIC_ROUTER_INVALIDATION	router-installation-invalidate
PUBSUB_SUBSCRIPTION_ROUTER_INVALIDATION	router-installation-invalidate-sub

For local dev / selfhosted: set PUBSUB_EMULATOR_HOST=localhost:8085 to route to the Pub/Sub emulator (same as Weave's distr/pubsub-emulator setup).

Test plan

• Deploy to staging with new env vars set, verify router boots and logs "Invalidation listener active"
• Rotate an installation key, confirm cache invalidation propagates without waiting for TTL
• Confirm no long-running idle Postgres sessions from the router after deploy

🤖 Generated with Claude Code

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 44cc1af. Configure here.}