feat: REST API with API keys, QR login, and web dashboard (v0.2.0)

- REST API (FastAPI): all data endpoints, sync (foreground/background),
  coverage stats, interactive docs at /docs
- API key system: exchange Xiaomi credentials for mif_sk_* keys
  (multi-account contexts, last-used tracking, prefix revocation);
  static key via MI_FITNESS_API_KEY, admin endpoints via MI_FITNESS_ADMIN_KEY
- QR code login: reverse-engineered Xiaomi universal QR flow
  (sid=xiaomiio), auto-issues API key after scan confirmation
- Built-in web dashboard (Flask): visual console for all endpoints,
  key management, QR login; reverse proxy with server-side key relay
- Standalone testpage/ console (python app.py, no install needed)
- Fix: pin mcp<2 (SDK 2.x removed Server.list_tools, broke startup)
- CI: install [all,dev] extras; docs: full technical documentation
  and QR login implementation guide

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -e ".[dev]"
+          pip install -e ".[all,dev]"
       - name: Ruff
         run: |
           python -m ruff check src tests