docs: add backend README sync rule to CLAUDE.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -92,6 +92,23 @@ This rule applies to all languages in this repo (Go and Dart).
 In Go, name variables to avoid shadowing the `error` built-in and the `context` package —
 use descriptive prefixes: `parseError`, `requestContext`, etc.
 
+## Backend Documentation
+
+**Rule:** `backend/README.md` must stay in sync with the code. When making backend changes,
+update the README if the change affects any of the following:
+
+- **Stack or dependencies** — new or removed libraries
+- **Environment variables** — new, renamed, or removed variables
+- **API endpoints** — new routes, changed paths or methods, removed endpoints
+- **Makefile targets** — new or removed `make` commands
+- **Project structure** — new top-level packages under `cmd/`, `internal/domain/`,
+  `internal/adapters/`, or `internal/infra/`
+- **Database schema** — new tables added to the high-level table list
+- **Import tools** — changes to `cmd/importoff` flags or behaviour
+
+The README does **not** need updating for: internal refactoring that doesn't change the
+public interface, bug fixes, test changes, or minor logic tweaks inside existing packages.
+
 ## Flutter Client Localisation
 
 **Rule:** Every UI string in `client/` must go through `AppLocalizations`.