feat: Settings can re-download the Discord images the None naming broke (3999)

The operator chose a clean re-download over relinking in place. The ~1,600 broken files can't be relinked reliably: their message ids are gone, and their sidecars collided.

Settings → Maintenance → "Repair Discord downloads" previews, then applies:
- Deletes every image whose path is `…/discord/None/<8 digits>_None_…`. Both the folder and the name are required, because that pair is only what the bug produced. It reuses cleanup_service.delete_images for the record and file deletes.
- Sweeps the leftover collided sidecars from those folders and removes the emptied folders.
- Only then clears gallery-dl's archive rows `discord%`, excluding `discordasset_%`. Upstream keys message attachments as `discord{message_id}_{num}`. Since the broken files lost their message ids, per-source forgetting is impossible. Every pre-fix Discord download is broken, and files fetched after the fix still exist on disk, so gallery-dl's `skip` won't re-fetch them.
- Arms a fresh backfill on every Discord source.

The apply defaults to preview at both the route and the task, runs on maintenance_long, and is never on a beat. The card uses the confirm-dialog pattern of AttachmentReclaimCard.

Supporting refactors, with no behaviour change:
- gallery_dl.archive_path() is the single definition of the archive location.
- source_service.arm_backfill() is the mutation start_backfill already did, now shared with the sync repair.

Tests (tests/test_discord_repair.py):
- The archive clear leaves other platforms and Discord assets alone, and counting mutates nothing.
- Case-twin artist folders are both found.
- The folder sweep works.
- An integration run shows only the broken image goes. A correctly named Discord file and a `None` folder under Patreon survive, and only Discord sources are re-armed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SHQB1YukL3VyvMK8rcbmV9

backend/app/services/source_service.py

@@ -158,6 +158,19 @@ def _is_app_managed(key: str) -> bool:
 BACKFILL_MAX_CHUNKS = 200
 
 
+def arm_backfill(source: Source) -> None:
+    """Arm a fresh run-until-done backfill on `source` (plan #693). Mutation
+    only — the caller commits. Shared by `SourceService.start_backfill` and the
+    sync Discord repair, so both clear exactly the same resume state."""
+    co = dict(source.config_overrides or {})
+    co["_backfill_state"] = "running"
+    for k in ("_backfill_cursor", "_backfill_cursor_stalls", "_backfill_chunks",
+              "_backfill_posts"):
+        co.pop(k, None)
+    source.config_overrides = co
+    source.backfill_runs_remaining = BACKFILL_MAX_CHUNKS
+
+
 class SourceService:
     def __init__(self, session: AsyncSession):
         self.session = session
@@ -473,13 +486,7 @@ class SourceService:
         )).scalar_one_or_none()
         if source is None:
             raise LookupError(f"source id={source_id} not found")
-        co = dict(source.config_overrides or {})
-        co["_backfill_state"] = "running"
-        for k in ("_backfill_cursor", "_backfill_cursor_stalls", "_backfill_chunks",
-                  "_backfill_posts"):
-            co.pop(k, None)
-        source.config_overrides = co
-        source.backfill_runs_remaining = BACKFILL_MAX_CHUNKS
+        arm_backfill(source)
         await self.session.commit()
         return await self._row_to_record(source)