Add README, changelog directory, and changelog convention to CLAUDE.md

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

CLAUDE.md

@@ -88,6 +88,20 @@ Browser → Vite dev server (:5173)
 2. Token stored in `localStorage`, attached to every request by the Axios interceptor
 3. Protected routes call `GET /api/users/me`; `get_current_user` dep validates the token on the server
 
+## Changelog convention
+
+Every time files are added or modified, append an entry to the relevant file in `changelog/` (one file per date, named `YYYY-MM-DD_<slug>.md`). If a file for today already exists, append to it rather than creating a new one.
+
+Each entry must include:
+- A heading with the date and a short description
+- `**Timestamp:**` in ISO-8601 format
+- A **Summary** sentence
+- A **Files Added / Modified / Deleted** list with one-line descriptions
+
+The `README.md` **Current State** section should be kept up to date whenever significant features are added or removed.
+
+---
+
 ### Adding a new resource
 
 1. Add ORM model in `app/models/`, import it in `app/models/__init__.py`