fix(thumbnails): surface backfill results + tighten validity check

Two coupled problems, operator-flagged 2026-06-01: "missing thumbnails
but triggering backfill found nothing."

1. **Backfill UI was a black box.** `POST /api/thumbnails/backfill`
   returned just `{celery_task_id}` and the admin card said
   "Enqueued." with no counts. There was no way to tell whether the
   scan found 0 candidates, 5000 candidates, or whether the worker
   even picked up the task. "Found nothing" was indistinguishable
   from a broken queue.

   Fix: refactor the scan into a sync helper (`_run_backfill_scan`)
   shared by the Celery task and the API endpoint. The API now runs
   the scan in an executor and returns `{scanned, enqueued, ok,
   regenerated}`. The actual thumbnail generation work still goes
   to the thumbnail Celery queue per row via
   `generate_thumbnail.delay()` — the scan itself is fast
   (SELECT id+thumbnail_path + a file.stat() per row).

2. **`_thumb_is_valid` accepted header-only corrupt files.** The
   magic-byte check passed for any 8-byte file starting with a JPEG
   or PNG header, including empty/truncated/zero-pad files that
   browsers render as broken. Backfill counted these as `ok` and
   never regenerated.

   Fix: also require file size ≥ MIN_THUMB_BYTES (256). Real
   thumbnails are minimum ~2KB even on solid-color sources; header-
   only corrupt files top out around 12 bytes. 256 is well above
   the corrupt floor and well below any legitimate thumbnail.

Plus the admin card now shows the per-run counts instead of
"Enqueued.":
  Scanned 5,432 · enqueued 3 (2 regenerated) · 5,429 ok

backend/app/api/thumbnails.py

@@ -1,5 +1,7 @@
 """Thumbnail admin API: backfill trigger."""
 
+import asyncio
+
 from quart import Blueprint, jsonify
 
 thumbnails_bp = Blueprint("thumbnails", __name__, url_prefix="/api/thumbnails")
@@ -7,7 +9,20 @@ thumbnails_bp = Blueprint("thumbnails", __name__, url_prefix="/api/thumbnails")
 
 @thumbnails_bp.route("/backfill", methods=["POST"])
 async def trigger_backfill():
-    from ..tasks.thumbnail import backfill_thumbnails
+    """Run the backfill scan synchronously, return the counts. The actual
+    thumbnail generation work is still off-loaded to the thumbnail Celery
+    queue via `generate_thumbnail.delay()` per missing row — so this
+    handler is fast even on a 100k-image library (a scan is just SELECT
+    id, thumbnail_path + a file.stat() per row, no heavy work).
 
-    r = backfill_thumbnails.delay()
-    return jsonify({"celery_task_id": r.id}), 202
+    Operator-flagged 2026-06-01: the previous fire-and-forget shape
+    returned `{celery_task_id}` only, so the admin UI had no idea whether
+    backfill found 0 or 5000 candidates — \"found nothing\" was
+    indistinguishable from \"the worker isn't picking up the task.\""""
+    from ..tasks.thumbnail import _run_backfill_scan
+
+    # Sync scan inside an executor so we don't block the event loop.
+    counts = await asyncio.get_running_loop().run_in_executor(
+        None, _run_backfill_scan,
+    )
+    return jsonify(counts), 200