feat: add is_trading filter to symbols API + hardcode secret paths + backfill script

- API /api/v1/symbols: add is_trading query param + is_trading field in response
- config.py: set default DB_PASSWORD_FILE=/run/secrets/db_pw.txt and ENCRYPTION_KEY_FILE=/run/secrets/enc_key.txt
- scripts/backfill_candles.py: new script to fetch 500 historical candles per symbol/timeframe for all is_trading=true symbols
- Cleared 2.88M non-trading candles from DB

backend/app/schemas/symbol.py

@@ -11,6 +11,7 @@ class SymbolResponse(BaseModel):
     base: str
     quote: str
     is_active: bool = True
+    is_trading: bool = False
 
 
 class SymbolSearchResponse(BaseModel):