feat(gallery): similar() hides presentation images (banner / editor screenshot)

Step 3 of milestone #128. Presentation-tagged images cluster on UI
chrome rather than content, so near any one of them they fill the whole
more-like-this grid. Excluded from candidates in the ONE whole-image
similarity surface (gallery similar mode, explore walk, and RelatedStrip
all ride GalleryService.similar) — the anchor itself may be a banner,
and wip stays surfaced: only the training pipelines exclude it.

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

backend/app/models/tag.py

@@ -43,6 +43,12 @@ class TagKind(StrEnum):
     # to keep historic tag rows queryable.
 
 
+# The seeded system tags (migration 0075). PRESENTATION tags additionally
+# hide from whole-image similarity results — they cluster on UI chrome, not
+# content. `wip` is real art: only the training pipelines exclude it.
+SYSTEM_TAG_NAMES = ("wip", "banner", "editor screenshot")
+PRESENTATION_SYSTEM_TAGS = ("banner", "editor screenshot")
+
 image_tag = Table(
     "image_tag",
     Base.metadata,

backend/app/services/gallery_service.py

@@ -23,7 +23,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import aliased
 
 from ..models import Artist, ImageProvenance, ImageRecord, Post, Source, Tag
-from ..models.tag import image_tag
+from ..models.tag import PRESENTATION_SYSTEM_TAGS, image_tag
 from .pagination import decode_cursor, encode_cursor
 from .tag_query import (
     fandom_join_alias,
@@ -693,9 +693,23 @@ class GalleryService:
         eff = _effective_date_col()
         stmt = select(ImageRecord, Post.post_date, eff.label("eff"))
         stmt = _outer_join_primary_post(stmt)
+        # Presentation images (banner / editor-screenshot system tags, #128)
+        # cluster on UI chrome rather than content, so near any one of them
+        # they'd fill the grid. Excluded from CANDIDATES only — the anchor
+        # itself may be a banner, and `wip` stays surfaced (real art; only
+        # the training pipelines exclude it).
+        presentation = (
+            select(image_tag.c.image_record_id)
+            .join(Tag, Tag.id == image_tag.c.tag_id)
+            .where(
+                Tag.is_system.is_(True),
+                Tag.name.in_(PRESENTATION_SYSTEM_TAGS),
+            )
+        )
         stmt = stmt.where(
             ImageRecord.siglip_embedding.is_not(None),
             ImageRecord.id != image_id,
+            ImageRecord.id.not_in(presentation),
         )
         stmt = _apply_scope(
             stmt, tag_ids=tag_ids, post_id=None,