fix(audit-g1): six one-liner drift fixes from 2026-06-02 audit

- BACKFILL_TIMEOUT_SECONDS 1800→1170: keep the subprocess timeout
  30s below Celery's hard time_limit=1200 so SIGKILL doesn't beat
  TimeoutExpired (matched the tick 870s/900s rationale). Backfill
  runs that hit the cap let the next tick continue via the archive.

- recover_interrupted_tasks orphan UPDATE now stamps finished_at;
  without it cleanup_old_tasks' WHERE finished_at<cutoff never
  reaped orphan-swept rows. recover_stalled_task_runs also now sets
  duration_ms (matches celery_signals.finalize's millisecond math).

- ExtensionService.quick_add_source arms NEW_SOURCE_BACKFILL_RUNS=3
  on Source creation, mirroring SourceService.create. Without it,
  Firefox quick-add on a creator with >20 unsynced posts walked the
  full feed until subprocess timeout. Renamed the constant from
  _NEW_SOURCE_BACKFILL_RUNS so it can be imported cross-module.

- gallery_dl.verify() accepts TIER_LIMITED as auth-success alongside
  NO_NEW_CONTENT — the download path (line 712) already does, and
  TIER_LIMITED proves auth reached the post and was told it was
  tier-gated. Verify endpoint previously showed red on this and
  prompted operators to rotate working cookies.

- prune_unused_tags now runs a single DELETE with the NOT-IN
  predicate find_unused_tags uses, instead of SELECT-ids →
  DELETE-WHERE-IN. Removes the psycopg 65535-param cliff that
  would have surfaced on a tag explosion (>65k unused tags).

- credentials.upload() reflects the returned record into the store
  cache (`.set(platform, rec)`) instead of evicting it; previously
  the card briefly rendered "no credential" between upload and
  loadAll().

backend/app/services/extension_service.py

@@ -16,6 +16,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..models import Artist, Source
 from ..utils.slug import slugify
+from .source_service import NEW_SOURCE_BACKFILL_RUNS
 
 
 class UnknownPlatformError(Exception):
@@ -204,9 +205,16 @@ class ExtensionService:
             return existing, False
         sp = await self.session.begin_nested()
         try:
+            # New subscription sources arm a few backfill runs so the
+            # first ticks walk the full history (otherwise gallery-dl's
+            # exit:20 short-circuits before the archive is built).
+            # Mirrors SourceService.create — without it, Firefox quick-
+            # add on a creator with >20 unsynced posts would surface
+            # as "check failed" with no diagnosis. Audit 2026-06-02.
             src = Source(
                 artist_id=artist_id, platform=platform,
                 url=url, enabled=True,
+                backfill_runs_remaining=NEW_SOURCE_BACKFILL_RUNS,
             )
             self.session.add(src)
             await self.session.flush()