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,