feat: add multi-symbol dashboard support (parser, API, UI)

- Add [SYMBOL] prefix to all bot/user_data_stream log messages
- Rewrite log_parser.py with multi-symbol regex, per-symbol state tracking, symbol columns in DB schema
- Rewrite dashboard_api.py with /api/symbols endpoint, symbol query params on all endpoints, SQL injection fix
- Update App.jsx with symbol filter tabs, multi-position display, dynamic header
- Add tests for log parser (8 tests) and dashboard API (7 tests)

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

src/user_data_stream.py

@@ -102,7 +102,7 @@ class UserDataStream:
             close_reason = "MANUAL"
 
         logger.info(
-            f"청산 감지({close_reason}): exit={exit_price:.4f}, "
+            f"[{self._symbol}] 청산 감지({close_reason}): exit={exit_price:.4f}, "
             f"rp={realized_pnl:+.4f}, commission={commission:.4f}, "
             f"net_pnl={net_pnl:+.4f}"
         )