fix(thumbnails): derive URL from stored thumbnail_path, not (sha256, mime)

The showcase/gallery/artist/series/post-feed APIs were constructing
thumbnail URLs from (sha256, mime). The MIME-based extension predicate
("png if image/png or image/gif else jpg") DISAGREED with the
thumbnailer's actual on-disk extension predicate ("png if alpha else
jpg"). Result: every PNG source without transparency 404'd (URL asked
.png, disk had .jpg); every WebP/AVIF source with transparency 404'd
(URL asked .jpg, disk had .png) — despite the thumbnail file existing
on disk.

The backfill task couldn't catch these because backfill checks the
ACTUAL thumbnail_path stored on the record (correct), not the URL the
browser fetches (broken derivation). So records with valid on-disk
thumbnails kept showing as broken in the UI no matter how many times
backfill ran.

Operator-flagged 2026-05-30: "the generate thumbnails function appears
to not catch all of the failed thumbnail cases" — turned out to not be
a backfill bug at all.

Fix: thumbnail_url now takes (thumbnail_path, sha256, mime) and returns
the stored path verbatim — Quart serves /images/* 1:1 from the volume
(frontend.py:20-36), so the URL IS the disk path. Falls back to the old
sha256+mime derivation only when thumbnail_path is NULL (thumbnailer
hasn't run yet); that URL will 404 in the browser until backfill catches
it, same as before the path was tracked.

All 8 callers updated: showcase_service, gallery_service (2 sites),
artist_service, series_service, post_feed_service, tag_directory_service,
artist_directory_service. The four sites whose query was raw-tuple now
also SELECT ImageRecord.thumbnail_path.

Net effect: every record that has a valid on-disk thumbnail will now
render correctly, regardless of which extension the thumbnailer chose,
without any DB migration or backfill rerun needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/post_feed_service.py

@@ -207,6 +207,7 @@ class PostFeedService:
                 ImageRecord.primary_post_id,
                 ImageRecord.sha256,
                 ImageRecord.mime,
+                ImageRecord.thumbnail_path,
                 func.row_number().over(
                     partition_by=ImageRecord.primary_post_id,
                     order_by=ImageRecord.id.asc(),
@@ -220,18 +221,18 @@ class PostFeedService:
         )
         stmt = select(
             ranked.c.id, ranked.c.primary_post_id,
-            ranked.c.sha256, ranked.c.mime, ranked.c.total,
+            ranked.c.sha256, ranked.c.mime, ranked.c.thumbnail_path, ranked.c.total,
         )
         if limit is not None:
             stmt = stmt.where(ranked.c.rn <= limit)
         rows = (await self.session.execute(stmt)).all()
 
         out: dict[int, dict] = {pid: {"thumbs": [], "more": 0} for pid in post_ids}
-        for img_id, pid, sha, mime, total in rows:
+        for img_id, pid, sha, mime, tp, total in rows:
             entry = out.setdefault(pid, {"thumbs": [], "more": 0})
             entry["thumbs"].append({
                 "image_id": img_id,
-                "thumbnail_url": thumbnail_url(sha, mime),
+                "thumbnail_url": thumbnail_url(tp, sha, mime),
                 "mime": mime,
             })
             # `total` is constant per partition; overflow = total - THUMBNAIL_LIMIT.