feat: add one-to-one box mode with duplicate bind check

Add box_mode field to BoxSaveRequest and DuplicateZongpaiBindError
exception. In one-to-one mode, prevent the same zongpai from being
packed into multiple boxes, returning 409 with existing box info.

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

app/schemas/box.py

@@ -32,6 +32,7 @@ class BoxSaveRequest(BaseModel):
     zongpai_no: str
     box_no: int
     quantity: int
+    box_mode: str | None = None  # one-to-one | one-to-many | many-to-one
 
     @field_validator("zongpai_no")
     @classmethod