feat(download): tick/backfill modes + partial-success classifier (plan #544)

Routine subscription polls walked the entire post history every tick
even when nothing had changed, because gallery-dl's default `skip: True`
continues iterating archived posts. A creator with ~550 archived posts
(Knuxy patreon) saturates the 870s wall-clock cap before completing,
even with zero downloads needed. Plus, a tier-limited run that
downloaded hundreds of files but ran out the clock should be a
warning, not an error.

Two coupled changes, both operator-flagged 2026-06-01:

* **Tick mode (default, cron polls).** New `TICK_SKIP_VALUE = "exit:20"`
  asks gallery-dl to exit after 20 contiguous archived items. Fresh
  subscriptions + new-content cases still walk normally; established
  subscription with zero new content exits in ~30s of HEAD requests
  instead of pegging the timeout. 20 (not 5) gives headroom against
  paywall warnings interleaving with archived items.
* **Backfill mode (explicit, operator-triggered).** Sticky for N runs
  via new `Source.backfill_runs_remaining` (alembic 0031). While > 0,
  downloads use `skip: True` + 1800s timeout. Auto-decrements per run
  with early-reset to 0 when a clean run finds zero files (queue
  drained). N defaults to 3 — multiple runs give the system enough
  budget to finish a deep walk across timeout boundaries. New
  `POST /api/sources/{id}/backfill` arms the source; "Deep scan"
  button on each SourceRow (chip shows remaining count) wires it.

Plus partial-success classifier: non-zero gallery-dl exit + ≥1 file
downloaded + no source-level error fires `ErrorType.PARTIAL`, which
download_service maps to `status=\"ok\"`. The run did real work; the
next tick continues via gallery-dl's archive. No more red events for
"timed out mid-walk after downloading 300 files."

Retires `SourceConfig.skip_existing` — skip value is now derived from
the source state and passed as a separate `skip_value` parameter
through download() / _build_config_for_source(). `GD_DEFAULTS` drops
the now-dead key (was inert data after this refactor).

Tests cover:
* tick + backfill skip-value emission in _build_config_for_source
* PARTIAL classifier branch + TIER_LIMITED-wins-over-PARTIAL ordering
* SourceService.set_backfill_runs validation + persistence
* /api/sources/{id}/backfill 200/400/404 paths
* download_service auto-decrement / auto-reset / tick-mode-no-touch
* PARTIAL → status=ok in the orchestrator (no consecutive_failures bump)

tests/test_source_service.py

@@ -169,3 +169,47 @@ async def test_list_hides_sidecar_synthetic_anchors(db):
     assert {s.url for s in everything} >= {
         "https://patreon.com/alice", "sidecar:patreon:alice",
     }
+
+
+# --- Plan #544: backfill counter -------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_set_backfill_runs_arms_source(db):
+    """The service method sets backfill_runs_remaining and returns the
+    updated record so the API can echo it back."""
+    artist = await _artist(db, "Alice")
+    svc = SourceService(db)
+    rec = await svc.create(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice",
+    )
+    assert rec.backfill_runs_remaining == 0
+    updated = await svc.set_backfill_runs(rec.id, 5)
+    assert updated.backfill_runs_remaining == 5
+
+    db_value = (await db.execute(
+        select(Source.backfill_runs_remaining).where(Source.id == rec.id)
+    )).scalar_one()
+    assert db_value == 5
+
+
+@pytest.mark.asyncio
+async def test_set_backfill_runs_rejects_out_of_range(db):
+    artist = await _artist(db, "Alice")
+    svc = SourceService(db)
+    rec = await svc.create(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice",
+    )
+    with pytest.raises(ValueError):
+        await svc.set_backfill_runs(rec.id, 0)
+    with pytest.raises(ValueError):
+        await svc.set_backfill_runs(rec.id, 11)
+
+
+@pytest.mark.asyncio
+async def test_set_backfill_runs_raises_when_source_missing(db):
+    svc = SourceService(db)
+    with pytest.raises(LookupError):
+        await svc.set_backfill_runs(99999, 3)