fix: unify error response format with error_code, message and domain fields

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

app/services/location_service.py

@@ -1,10 +1,17 @@
-from fastapi import HTTPException
 from sqlalchemy.orm import Session
 
 from app.models.finished_goods import FinishedGoodsLocation
 from app.schemas.location import LocationRequest
 
 
+class DuplicateLocationError(Exception):
+    """总排号已存在货位记录。"""
+
+    def __init__(self, location_code: str, registered_at: str):
+        self.location_code = location_code
+        self.registered_at = registered_at
+
+
 def register_location(db: Session, req: LocationRequest) -> FinishedGoodsLocation:
     existing = (
         db.query(FinishedGoodsLocation)
@@ -12,16 +19,9 @@ def register_location(db: Session, req: LocationRequest) -> FinishedGoodsLocatio
         .first()
     )
     if existing:
-        raise HTTPException(
-            status_code=409,
-            detail={
-                "error_code": "DUPLICATE_LOCATION",
-                "message": "该总排号已存在货位记录",
-                "detail": {
-                    "existing_location": existing.location_code,
-                    "created_at": existing.created_at.isoformat(),
-                },
-            },
+        raise DuplicateLocationError(
+            location_code=existing.location_code,
+            registered_at=existing.created_at.isoformat(),
         )
 
     record = FinishedGoodsLocation(