docs: fix stale migration and endpoint counts, add missing endpoints

- Update migration count from 11 to 13 across all docs (012_bom_source,
  013_move_cost_sourcing_to_props)
- Update endpoint count from 75 to 78 across all docs
- Add 3 missing endpoints to SPECIFICATION.md section 11.1:
  GET /api/events (SSE), GET /api/items/by-uuid/{uuid},
  POST /api/items/{pn}/bom/merge
- Add migrations 012 and 013 to STATUS.md table
- Fix migration 010 description (sourcing_link and standard_cost moved
  to revision properties in 013)

README.md

@@ -25,7 +25,7 @@ silo/
 │   ├── silo/              # CLI tool
 │   └── silod/             # API server
 ├── internal/
-│   ├── api/               # HTTP handlers and routes (75 endpoints)
+│   ├── api/               # HTTP handlers and routes (78 endpoints)
 │   ├── auth/              # Authentication (local, LDAP, OIDC)
 │   ├── config/            # Configuration loading
 │   ├── db/                # PostgreSQL repositories