feat(subscriptions): newly added enabled sources start in backfill mode

A freshly created subscription has no gallery-dl archive yet, so the
first poll in tick mode would walk forever — exit:20 doesn't trip
until 20 contiguous archive-hits, and there are none. The wall-clock
cap kicks in mid-walk, and the partial-success classifier (plan #544)
gracefully labels it status=ok, but the operator still wonders why
their new subscription isn't grabbing the back-catalog.

Pre-arm `backfill_runs_remaining = 3` on create() when `enabled=True`.
Same default as the manual "Deep scan" button, same auto-decrement
and auto-reset rules — once the queue drains (clean exit + zero new
files) or the budget runs out, tick mode resumes naturally.

Disabled sources (sidecar synthetics with `url='sidecar:<platform>:<slug>'`
that arrive disabled = False, or operator-added sources that start
disabled deliberately) skip the pre-arm — they're never polled, no
budget to waste.

tests/test_source_service.py

@@ -213,3 +213,31 @@ 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)
+
+
+@pytest.mark.asyncio
+async def test_new_enabled_source_starts_in_backfill_mode(db):
+    """Plan #544 follow-up: freshly added enabled sources have no archive
+    yet, so the first few polls would blow the wall-clock cap in tick
+    mode. Pre-arm backfill so the initial walks use the longer timeout."""
+    artist = await _artist(db, "Alice")
+    svc = SourceService(db)
+    rec = await svc.create(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice-new",
+    )
+    assert rec.backfill_runs_remaining == 3
+
+
+@pytest.mark.asyncio
+async def test_new_disabled_source_skips_backfill(db):
+    """Disabled sources (incl. sidecar synthetics that arrive disabled) are
+    never polled, so don't burn a backfill budget on them."""
+    artist = await _artist(db, "Alice")
+    svc = SourceService(db)
+    rec = await svc.create(
+        artist_id=artist.id, platform="patreon",
+        url="https://patreon.com/alice-disabled",
+        enabled=False,
+    )
+    assert rec.backfill_runs_remaining == 0