Backend now fully supports negative consumption

Updated integration tests to verify this.

spoolman/database/spool.py

@@ -4,6 +4,7 @@ from datetime import datetime, timezone
 from typing import Optional
 
 import sqlalchemy
+from sqlalchemy import case
 from sqlalchemy.exc import NoResultFound
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import contains_eager, joinedload
@@ -149,7 +150,12 @@ async def use_weight_safe(db: AsyncSession, spool_id: int, weight: float) -> Non
     await db.execute(
         sqlalchemy.update(models.Spool)
         .where(models.Spool.id == spool_id)
-        .values(used_weight=models.Spool.used_weight + weight),
+        .values(
+            used_weight=case(
+                (models.Spool.used_weight + weight >= 0.0, models.Spool.used_weight + weight),  # noqa: PLR2004
+                else_=0.0,  # Set used_weight to 0 if the result would be negative
+            ),
+        ),
     )