feat(server): add POST /attachment/box with real-evidence flip + cap

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

app/main.py

@@ -36,6 +36,7 @@ from app.services.attachment_service import (
     AlreadyOffShelfAttachmentError,
     CategoryHasBoxesError,
     CategoryNotFoundError,
+    DuplicateAttachmentBoxItemError,
     DuplicateAttachmentLocationError,
 )
 
@@ -231,6 +232,21 @@ async def already_off_shelf_attachment_handler(
     )
 
 
+@app.exception_handler(DuplicateAttachmentBoxItemError)
+async def duplicate_attachment_box_handler(
+    request: Request, exc: DuplicateAttachmentBoxItemError
+):
+    return JSONResponse(
+        status_code=409,
+        content={
+            "error_code": "DUPLICATE_BOX_NO",
+            "message": f"排产号 {exc.paichan_no} 下箱号 {exc.box_no} 已存在",
+            "paichan_no": exc.paichan_no,
+            "box_no": exc.box_no,
+        },
+    )
+
+
 @app.get("/")
 async def root():
     return {"message": "Welcome to CargoTrace API"}