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>

tests/test_patreon_downloader.py

@@ -397,6 +397,25 @@ def test_skips_do_not_pace(tmp_path, monkeypatch):
     assert slept == []
 
 
+def test_download_post_honors_should_stop_mid_post(tmp_path):
+    """The time-box is polled BEFORE each media item, so a heavy post can't run
+    the whole chunk past its budget (Pocketacer soft-limit, 2026-06-07)."""
+    dl = PatreonDownloader(
+        images_root=tmp_path, cookies_path=None, validate=False,
+        session=_FakeSession(),
+    )
+    items = [_img("a.png"), _img("b.png"), _img("c.png")]
+    polls = {"n": 0}
+
+    def _stop():
+        polls["n"] += 1
+        return polls["n"] > 1  # allow the first item, stop before the second
+
+    outcomes = dl.download_post(_post(), items, "artist-x", should_stop=_stop)
+    assert len(outcomes) == 1  # only the first item ran; the rest re-fetch later
+    assert outcomes[0].status == "downloaded"
+
+
 def test_media_429_retried_then_succeeds(tmp_path, monkeypatch):
     slept: list[float] = []
     monkeypatch.setattr(pd_mod.time, "sleep", lambda s: slept.append(s))