feat: update docs for token efficient usage

backend/src/core/CLAUDE.md

@@ -0,0 +1,20 @@
+# backend/src/core/
+
+## Files
+
+| File | What | When to read |
+| ---- | ---- | ------------ |
+| `README.md` | Core module index with code examples | Understanding core infrastructure |
+
+## Subdirectories
+
+| Directory | What | When to read |
+| --------- | ---- | ------------ |
+| `auth/` | Authentication utilities | JWT handling, user context |
+| `config/` | Configuration loading (env, database, redis) | Environment setup, connection pools |
+| `logging/` | Winston structured logging | Log configuration, debugging |
+| `middleware/` | Fastify middleware | Request processing, user extraction |
+| `plugins/` | Fastify plugins (auth, error, logging) | Plugin registration, hooks |
+| `scheduler/` | Job scheduling infrastructure | Scheduled tasks, cron jobs |
+| `storage/` | Storage abstraction and adapters | File storage, S3/filesystem |
+| `user-preferences/` | User preferences data and migrations | User settings storage |