Verify deletion actually removed the file before reporting success

MeTube's /delete only unlinks the file when it's configured with
DELETE_FILE_ON_TRASHCAN=true; otherwise it just drops the entry from its
own "done" list and returns {"status": "ok"} regardless -- we were trusting
that response and marking the job "deleted" (offering a re-download) while
the file was still sitting on mediaVM's disk the whole time.

Now HEAD-check the media_url right after the delete call. If the file is
still reachable, leave the job's status untouched (still "completed", still
playable) and surface a clear 409 explaining MeTube's own config is why,
rather than lying about local state.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/services/download_jobs.py

@@ -19,6 +19,15 @@ class DeleteNotAllowed(Exception):
     pass
 
 
+class DeleteDidNotRemoveFile(Exception):
+    """MeTube accepted the /delete request but the file is still reachable
+    afterwards -- its DELETE_FILE_ON_TRASHCAN setting is very likely off on
+    that instance, which is outside this app's control (see AGENTS.md
+    constraint 11: never touch MeTube's own config/source)."""
+
+    pass
+
+
 def get_latest_job(db: Session, video_id: int) -> DownloadJob | None:
     return (
         db.query(DownloadJob)
@@ -69,7 +78,18 @@ def delete_local_copy(db: Session, video: Video) -> DownloadJob:
     if not job.metube_job_id:
         raise DeleteNotAllowed("Missing MeTube job id, cannot request deletion")
 
-    MeTubeClient().delete_download(job.metube_job_id)
+    client = MeTubeClient()
+    client.delete_download(job.metube_job_id)
+
+    # MeTube's /delete only unlinks the file if it's configured with
+    # DELETE_FILE_ON_TRASHCAN=true -- otherwise it just drops the entry from
+    # its own "done" list and the file stays put. We don't control that
+    # instance's config, so verify rather than trust the "ok" response.
+    if job.media_url and client.check_media(job.media_url):
+        raise DeleteDidNotRemoveFile(
+            "MeTube removed the entry but the file is still on disk "
+            "(its DELETE_FILE_ON_TRASHCAN setting is likely off)"
+        )
 
     job.status = "deleted"
     job.media_url = None