feat(ml): training hygiene — system-tagged images are absent from other concepts training

Step 2 of milestone #128. _hygiene_excluded_ids (training_data.py) is the
one shared predicate: images carrying any system tag are dropped from
every OTHER concepts head training — not positives (a rough wip tagged
as a character drags the head toward generic-sketch) and not rejection
or sampled negatives (a wip OF character X is not evidence against X).
A system tags own head trains on them unfiltered; that is what makes
auto-flagging banners work. Selection is split out of train_head as the
sklearn-free head_training_ids so CI (no sklearn) can pin the behavior.

CCIP: reference prototypes skip hygiene-tagged images — a faceless wip
figure region must never become an identity reference — and the ref
cache signature now counts hygiene applications, since tagging an image
wip changes the reference set without touching character/region counts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/ml/ccip.py

@@ -62,6 +62,18 @@ def _single_character_images():
     )
 
 
+def _hygiene_tagged_images():
+    """Subquery of image ids carrying any SYSTEM tag (wip / banner / editor
+    screenshot). Training hygiene (#128): such images never contribute
+    reference prototypes — a faceless wip's figure region would otherwise
+    become an identity reference for the character it's tagged with."""
+    return (
+        select(image_tag.c.image_record_id)
+        .join(Tag, Tag.id == image_tag.c.tag_id)
+        .where(Tag.is_system.is_(True))
+    )
+
+
 async def _ref_signature(session: AsyncSession) -> tuple:
     n_tags = (
         await session.execute(
@@ -79,7 +91,17 @@ async def _ref_signature(session: AsyncSession) -> tuple:
             )
         )
     ).one()
-    return (n_tags, n_regs, max_id)
+    # Hygiene applications must invalidate too: tagging an image `wip` changes
+    # the reference set without touching character-tag or region counts.
+    n_hygiene = (
+        await session.execute(
+            select(func.count())
+            .select_from(image_tag)
+            .join(Tag, Tag.id == image_tag.c.tag_id)
+            .where(Tag.is_system.is_(True))
+        )
+    ).scalar_one()
+    return (n_tags, n_regs, max_id, n_hygiene)
 
 
 async def character_references(session: AsyncSession) -> dict[int, list]:
@@ -102,6 +124,9 @@ async def character_references(session: AsyncSession) -> dict[int, list]:
             .where(ImageRegion.kind.in_(_FIGURE_KINDS))
             .where(ImageRegion.ccip_embedding.is_not(None))
             .where(ImageRegion.image_record_id.in_(_single_character_images()))
+            .where(
+                ImageRegion.image_record_id.not_in(_hygiene_tagged_images())
+            )
         )
     ).all()
     refs: dict[int, list] = {}