chore: update docs

backend/src/features/ownership-costs/CLAUDE.md

@@ -0,0 +1,17 @@
+# ownership-costs/
+
+## Files
+
+| File | What | When to read |
+| ---- | ---- | ------------ |
+| `README.md` | Feature documentation | Understanding cost tracking |
+| `index.ts` | Feature barrel export | Importing ownership-cost services |
+
+## Subdirectories
+
+| Directory | What | When to read |
+| --------- | ---- | ------------ |
+| `api/` | HTTP endpoints and routes | API changes |
+| `domain/` | Business logic, services, types | Core cost calculation logic |
+| `data/` | Repository, database queries | Database operations |
+| `migrations/` | Database schema | Schema changes |