Unified Resource Event System

Description

Refactor the event system from 40+ specialized event variants to a unified generic resource event architecture. This eliminates boilerplate and enables horizontal scaling - adding new resources requires zero event handling code changes.

Current Problem

• 40+ event variants in core/src/infra/event/mod.rs
• Manual event emission scattered across codebase (easy to forget)
• Adding new resource = new event variant + client code changes
• No type safety between events and resources

Solution

• Generic ResourceChanged, ResourceBatchChanged, ResourceDeleted events
• TransactionManager emits automatically (no manual emission)
• Client type registries handle deserialization generically
• Infrastructure events remain specific (CoreStarted, Job, etc.)

Implementation Steps

1. Define new Event struct with EventEnvelope and EventKind
2. Add ResourceChanged and related variants to EventKind
3. Update TransactionManager to emit resource events automatically
4. Keep infrastructure events as specific variants
5. Mark old event variants as #[deprecated]
6. Migrate Albums/Tags/Locations to new events (parallel systems)
7. Remove old variants after full migration

Acceptance Criteria

• New event structure defined
• EventEnvelope includes id, timestamp, library_id, sequence
• ResourceChanged auto-emitted by TransactionManager
• Infrastructure events (Job, CoreStarted) preserved
• No breaking changes (parallel systems initially)
• Documentation updated

Migration Strategy

• Phase 1: Additive (both old and new events)
• Phase 2: Parallel (new resources use unified events only)
• Phase 3: Deprecation (mark old events deprecated)
• Phase 4: Cleanup (remove old events)

References

• docs/core/events.md - Complete specification
• Current: core/src/infra/event/mod.rs