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/api/v1/symbols.py

@@ -21,11 +21,12 @@ router = APIRouter(prefix="/symbols", tags=["symbols"])
 async def list_symbols(
     exchange: Optional[str] = Query(None, description="Filter by exchange name"),
     active_only: bool = Query(True, description="Only return active symbols"),
+    is_trading: Optional[bool] = Query(None, description="Filter by trading status (top-100 bases)"),
     limit: int = Query(50, ge=1, le=1000, description="Max symbols to return"),
     offset: int = Query(0, ge=0, description="Offset for pagination"),
     db: AsyncSession = Depends(get_db_session),
 ) -> list[SymbolResponse]:
-    """Return symbols with pagination, optionally filtered by exchange and/or active status."""
+    """Return symbols with pagination, optionally filtered by exchange, active status, and trading flag."""
     query = select(Symbol).options(joinedload(Symbol.exchange))
 
     if exchange:
@@ -34,6 +35,8 @@ async def list_symbols(
         )
     if active_only:
         query = query.where(Symbol.is_active == True)  # noqa: E712
+    if is_trading is not None:
+        query = query.where(Symbol.is_trading == is_trading)
 
     query = query.limit(limit).offset(offset)
     result = await db.execute(query)
@@ -47,6 +50,7 @@ async def list_symbols(
             base=s.base,
             quote=s.quote,
             is_active=s.is_active,
+            is_trading=s.is_trading,
         )
         for s in symbols
     ]
@@ -82,6 +86,7 @@ async def search_symbols(
                 base=s.base,
                 quote=s.quote,
                 is_active=s.is_active,
+                is_trading=s.is_trading,
             )
             for s in symbols
         ]