fix(patreon): enforce the backfill time-box mid-post (stop overrunning to the soft limit)

A backfill chunk's time-box (BACKFILL_CHUNK_SECONDS=600) was only checked
between POSTS, but download_post downloads ALL of one post's media
synchronously — so a single media-heavy post could run the chunk far past 600s,
all the way to the Celery soft time limit (1350s), where it was killed and
finalized as error (Pocketacer, event #41330: ran the full 22.5 min).

download_post now polls a should_stop() deadline BEFORE each media item and the
engine passes `now - start >= time_budget_seconds`, so a heavy post stops at the
budget and the remaining media (never marked seen) re-fetch next chunk. Bounds
chunk overrun to one media download instead of one whole post.

Also genericized the soft-limit salvage message — it claimed the "gallery-dl
subprocess" failed, which is wrong for a native Patreon walk; it now describes
the time-budget overrun + per-page checkpoint resume in platform-neutral terms.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/patreon_downloader.py

@@ -190,6 +190,7 @@ class PatreonDownloader:
         artist_slug: str,
         *,
         is_seen: Callable[[object], bool] = lambda m: False,
+        should_stop: Callable[[], bool] = lambda: False,
     ) -> list[MediaOutcome]:
         """Download every media item of one post; return per-item outcomes.
 
@@ -198,11 +199,19 @@ class PatreonDownloader:
         yt-dlp for video), writes the sidecar for each freshly-downloaded item,
         validates, and returns outcomes. Resilient: one media's failure yields
         an "error" outcome for that item; the rest proceed.
+
+        `should_stop()` is polled BEFORE each media item: a media-dense post can
+        otherwise run a backfill chunk far past its time-box (the engine only
+        re-checks the budget between posts), so we honour the deadline mid-post
+        and return the items done so far — the rest re-fetch next chunk (they
+        were never marked seen). Bounds chunk overrun to one media download.
         """
         post_dir = self.images_root / artist_slug / "patreon" / _post_dir_name(post)
         outcomes: list[MediaOutcome] = []
 
         for i, media in enumerate(media_items, start=1):
+            if should_stop():
+                break
             try:
                 outcomes.append(
                     self._download_one(post, media, post_dir, artist_slug, i, is_seen)