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.

backend/app/services/source_service.py

@@ -87,6 +87,11 @@ class SourceRecord:
 
 _EDITABLE = {"enabled", "url", "config_overrides", "check_interval_override", "platform"}
 
+# Plan #544 follow-up: newly created enabled sources pre-arm backfill so
+# their first N polls walk gallery-dl's full post history with the longer
+# timeout (matches the manual "Deep scan" button's default).
+_NEW_SOURCE_BACKFILL_RUNS = 3
+
 
 class SourceService:
     def __init__(self, session: AsyncSession):
@@ -204,10 +209,21 @@ class SourceService:
             select(func.count(Source.id)).where(Source.artist_id == artist_id)
         )).scalar_one()
 
+        # Plan #544 follow-up: a freshly added subscription has no archive
+        # yet, so the first few polls would walk the full post history in
+        # tick mode and trip exit:20 after ~20 contiguous archive hits —
+        # except there are none yet, so tick mode would walk forever and
+        # blow the wall-clock cap. Pre-arm backfill so the initial syncs
+        # use the longer timeout + skip:True walk. Tick mode resumes once
+        # the budget is spent or the queue drains.
+        # Disabled sources (incl. sidecar synthetics, url='sidecar:...')
+        # are never polled, so leave their counter at 0.
+        backfill_runs = _NEW_SOURCE_BACKFILL_RUNS if enabled else 0
         source = Source(
             artist_id=artist_id, platform=platform, url=url,
             enabled=enabled, config_overrides=config_overrides,
             check_interval_override=check_interval_override,
+            backfill_runs_remaining=backfill_runs,
         )
         self.session.add(source)
         try: