feat(system-tags): process vs chrome groups + WIP provisional auto-apply (#1464)

Backend for the system-tag behavior refactor (milestone #157). editor screenshot
moves from chrome (hidden) to the PROCESS group (shown, like wip); wip+editor gain
provisional auto-apply so they stop needing endless manual identification —
without a runaway loop.

- tag.py: split PRESENTATION_SYSTEM_TAGS → CHROME_SYSTEM_TAGS (banner) +
  PROCESS_SYSTEM_TAGS (wip, editor screenshot).
- heads.py: generalize presentation_auto_apply_sweep → system_tag_auto_apply_sweep
  (mode chrome|process). Same Guard 1 (skip human/confirmed) + Guard 2 (ring-loud
  conflict → PresentationReview). process mode uses source 'process_auto' and does
  NOT hide (hide is a gallery-query effect of group membership).
- training_data._AUTO_SOURCES += 'process_auto' → the head never trains on its own
  auto-applied output; only wip_title/manual train it (the runaway break).
- ml_settings: process_auto_apply_enabled (OFF, opt-in) + threshold + conflict
  threshold. presentation_review.mode ('chrome'|'process'). Migration 0086.
- gallery_service: default-hide reads CHROME only (editor now shows); Explore
  neighbors exclude the whole PROCESS group.
- tasks/ml + celery beat: scheduled_process_auto_apply (daily, opt-in); prune
  covers both modes.
- api: ml_admin process_* CRUD+validation; hidden-review returns mode.
- tests: rename chrome sweep calls; new test_process_auto_apply (apply, guards,
  mode flag, no-self-train); gallery test asserts editor now visible.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/ml/training_data.py

@@ -29,7 +29,10 @@ from ...models.tag import image_tag
 # a CCIP reference) unless the operator confirms them (milestone 139). Keeping
 # auto-applied predictions out of training is what makes them "soft" — a misfire
 # can't reinforce itself, so the retraction sweep can actually drop it.
-_AUTO_SOURCES = ("head_auto", "ccip_auto", "ml_auto", "presentation_auto")
+# `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).
+_AUTO_SOURCES = ("head_auto", "ccip_auto", "ml_auto", "presentation_auto", "process_auto")
 
 
 def _hygiene_excluded_ids(session: Session) -> set[int]: