fix: move instruments.json from data/ to config/ — Docker volume was masking /app/data

The docker-compose mounts db_data named volume at /app/data which hid the
instruments.json file baked into the image. Moving to /app/config which is
not volume-overlaid resolves the FileNotFoundError on startup.

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

backend/services/instrument_service.py

@@ -15,7 +15,7 @@ from datetime import datetime, date
 logger = logging.getLogger(__name__)
 
 # ── Config loading ─────────────────────────────────────────────────────────────
-CONFIG_PATH = Path(__file__).parent.parent / "data" / "instruments.json"
+CONFIG_PATH = Path(__file__).parent.parent / "config" / "instruments.json"
 _configs: Optional[Dict[str, Any]] = None
 
 # In-memory narrative cache: key = (instrument_id, iso_date) → str