fix(ownership): validate reason+priority in QueueItemCreate to return 422 not 500

POST /ownership/queue with an invalid reason or priority was silently
passing Pydantic and crashing at the DB layer (PostgreSQL enum type
mismatch → 500). Added @field_validator for both fields, matching the
existing validators in QueueItemPatch.

backend/app/schemas/ownership_queue_schema.py

@@ -100,6 +100,28 @@ class QueueItemCreate(BaseModel):
     assigned_to: Optional[UUID] = None
     due_date: Optional[datetime] = None
 
+    @field_validator("reason")
+    @classmethod
+    def validate_reason(cls, v):
+        from app.models.ownership_queue import QueueReason
+        try:
+            QueueReason(v)
+        except ValueError:
+            valid = [e.value for e in QueueReason]
+            raise ValueError(f"Invalid reason '{v}'. Must be one of: {valid}")
+        return v
+
+    @field_validator("priority")
+    @classmethod
+    def validate_priority(cls, v):
+        from app.models.ownership_queue import QueuePriority
+        try:
+            QueuePriority(v)
+        except ValueError:
+            valid = [e.value for e in QueuePriority]
+            raise ValueError(f"Invalid priority '{v}'. Must be one of: {valid}")
+        return v
+
 
 class QueueItemOut(BaseModel):
     model_config = ConfigDict(from_attributes=True)