docs: update project plan to reflect actual implementation status
Marked Phase 0 and Phase 1 items as complete based on codebase audit:
Phase 0 (Foundation):
- Forum Service Account setup (ATB-5)
- PDS spike script validation (ATB-6)
Phase 1 (AppView Core):
- Database schema with 7 tables (ATB-7)
- Firehose subscription with Jetstream (ATB-9)
- Record indexer for all types (ATB-10)
- Read/Write API scaffolding (ATB-11, ATB-12 - in progress)
Linear issues ATB-10 and ATB-11 updated to match reality.