feat: retire the sketch/doodle WIP title tier, and the review strip asks "Is this a WIP?" (milestone 430)

The soft tier (#1474) tagged `wip` on any post titled sketch, doodle or
scribble: 6,096 of the library's 8,876 wip tags. Its conflict audit flagged
most of them, because finished art scores >= 0.5 on some content head,
which filled the Gallery strip with 2,086 cards. A "sketch" is usually
finished work, so the operator retired the tier. The artist's own
"WIP" / "work in progress" title rule and human wip tags stay.

- Removed: the soft matcher, source and prefilter; the importer and backfill
  soft branches; soft_wip_conflict_audit with its task and beat; the
  wip_soft_title_tagging_enabled setting, API field and toggle; and
  wip_title_soft from _AUTO_SOURCES.
- Migration 0112: a soft tag the operator confirmed, or kept from the strip,
  becomes `manual`. Every other soft tag is deleted, along with the open
  review cards whose tag is gone. The column is dropped.
- Review strip (#4424): each card asks "Is this a WIP?" (or banner, and so on),
  and the buttons read "Is a WIP" / "Is not a WIP". The content tag it also
  scored on is shown as the reason it was flagged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/tasks/ml.py

@@ -620,24 +620,6 @@ def scheduled_process_auto_apply() -> str:
     return f"applied={result['n_applied']} flagged={result['n_flagged']}"
 
 
-@celery.task(
-    name="backend.app.tasks.ml.scheduled_soft_wip_conflict_audit",
-    soft_time_limit=1800, time_limit=2100,
-)
-def scheduled_soft_wip_conflict_audit() -> str:
-    """Ring-loud audit over the SOFT WIP-title cohort (#1474) — flag sketch/doodle
-    auto-tags that ALSO look like real content for review. No-op when there are no
-    content heads; idempotent (already-flagged images skipped). Runs regardless of
-    the process-sweep toggle, since soft-title tags come from the importer, not that
-    sweep. Wall-clock bounded by the task time limits."""
-    from ..services.ml.heads import soft_wip_conflict_audit
-
-    SessionLocal = _sync_session_factory()
-    with SessionLocal() as session:
-        result = soft_wip_conflict_audit(session)
-    return f"scanned={result['n_scanned']} flagged={result['n_flagged']}"
-
-
 @celery.task(name="backend.app.tasks.ml.prune_presentation_reviews")
 def prune_presentation_reviews() -> str:
     """Retention (rule 89): drop RESOLVED presentation-review flags older than 30