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/services/ml/training_data.py

@@ -32,11 +32,8 @@ from ...models.tag import image_tag
 # `process_auto` (#1464): wip/editor screenshot applied by the process sweep are
 # ALSO provisional — the head must learn only from title (`wip_title`) + manual
 # labels, never its own auto-applied output, or it would runaway (operator 2026-07-12).
-# `wip_title_soft` (#1474): the soft title tier (sketch/doodle) is LOW-precision, so
-# it's provisional too — a finished piece titled "sketch" must not train the wip head.
 _AUTO_SOURCES = (
     "head_auto", "ccip_auto", "ml_auto", "presentation_auto", "process_auto",
-    "wip_title_soft",
 )