Add zone subscription endpoints and some extra fixes

Summary

• Add federation zone sync endpoints (POST/GET/DELETE /{federationContextId}/zones)
• Merge local SRM zones with cached federated zones in /edge-cloud-zones
• Aggregate app instances across all federations in GET /appinstances
• Support deleting federated app instances and apps when SRM returns 404

Testing

• Unit tests for zone caching and federation endpoints

• End-to-end tested: partner registration → zone sync → app onboarding → instance lifecycle

  output.mp4