docs: Add centralized API architecture plan

CRITICAL ARCHITECTURAL DECISION:
- ONLY api worker has fetch handlers
- All other workers are RPC-only (WorkerEntrypoint)
- Centralizes security, logging, routing, monitoring

Current State:
- api worker exists with routing infrastructure
- Domain routing via Workers Assets (domain-routes.json)
- 20+ workers have fetch handlers that need removal

Implementation Plan (4 weeks):
Week 1: Generate routing table from sdk/.domains.tsv (105 domains)
Week 2: Convert 8 core services to RPC-only
Week 3: Update API routing, handle special cases
Week 4: Testing and deployment

Benefits:
- Single auth/rate limit enforcement point
- Centralized logging and monitoring
- Simpler services (no HTTP logic)
- Faster RPC vs HTTP

Author: nathanclevenger