feat: Implement cancellation of stale generations in the service and repository, along with a new test.

repos/generation_repo.py

@@ -1,4 +1,5 @@
 from typing import Optional, List
+from datetime import datetime, timedelta, UTC
 
 from PIL.ImageChops import offset
 from bson import ObjectId
@@ -85,3 +86,20 @@ class GenerationRepo:
             generation["id"] = str(generation.pop("_id"))
             generations.append(Generation(**generation))
         return generations
+
+    async def cancel_stale_generations(self, timeout_minutes: int = 60) -> int:
+        cutoff_time = datetime.now(UTC) - timedelta(minutes=timeout_minutes)
+        res = await self.collection.update_many(
+            {
+                "status": GenerationStatus.RUNNING,
+                "created_at": {"$lt": cutoff_time}
+            },
+            {
+                "$set": {
+                    "status": GenerationStatus.FAILED,
+                    "failed_reason": "Timeout: Execution time limit exceeded",
+                    "updated_at": datetime.now(UTC)
+                }
+            }
+        )
+        return res.modified_count