chore: remove vibe-coding artifacts from tracking, sync notes and obsidian config

- Untrack .claude/launch.json (Claude Code config)
- Add .claude/, .mcp.json, CLAUDE.md, and AI-generated CSVs to .gitignore
- Add _notes/ vault (architecture, API, tasks, bugs, infra docs)
- Add .obsidian/ plugin/vault config (workspace state excluded)
- Sync CHANGELOG.md

_notes/04 - API & Backend/API Notes.md

@@ -0,0 +1,46 @@
+# API & Backend Notes
+
+**API client location:** `lib/core/api/`
+
+## Overview
+> Document the backend API base URL, auth method, and overall structure here.
+
+## Authentication
+> e.g. Bearer token, JWT, OAuth2 — add details as you discover them
+
+## Endpoints
+
+### Auth
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| POST | `/auth/login` | User login |
+| POST | `/auth/register` | User registration |
+| POST | `/auth/logout` | Logout |
+
+### Events
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/events` | List events |
+| GET | `/events/:id` | Event detail |
+| POST | `/events/:id/book` | Book event |
+
+### Reviews
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/events/:id/reviews` | Get reviews |
+| POST | `/events/:id/reviews` | Submit review |
+
+### Gamification
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/leaderboard` | Get leaderboard |
+| GET | `/users/:id/achievements` | User achievements |
+
+> Update endpoints above as you explore `lib/core/api/`
+
+## Error Handling
+> Document API error codes and how interceptors handle them
+
+## Related
+- [[02 - Features/Auth]]
+- [[02 - Features/Events]]