feat(gallery): default-hide presentation chrome (banner/editor screenshot) (#141 step 1)

The default gallery + facets now implicitly exclude images carrying a
presentation system tag (banner / editor screenshot), reusing the tag-scope
EXISTS machinery. Suppressed when the operator explicitly filters FOR a
presentation tag OR passes include_hidden (the Hidden view — step 2). `wip` is
NOT hidden (real, in-progress art). include_hidden threaded through
scroll/timeline/jump_cursor/facets + the gallery API _parse_filters. Test covers
default-hide, include_hidden, explicit-filter-shows, and wip-stays-visible.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/gallery.py

@@ -77,6 +77,9 @@ def _parse_filters():
     platform = request.args.get("platform") or None
     untagged = request.args.get("untagged") in ("1", "true", "yes")
     no_artist = request.args.get("no_artist") in ("1", "true", "yes")
+    # Show the presentation chrome (banner / editor screenshot) that the default
+    # gallery hides — the Hidden view sets this (milestone 141).
+    include_hidden = request.args.get("include_hidden") in ("1", "true", "yes")
     date_from = _parse_date(request.args.get("date_from"))
     date_to = _parse_date(request.args.get("date_to"))
     if date_to is not None:
@@ -88,6 +91,7 @@ def _parse_filters():
         "platform": platform,
         "untagged": untagged, "no_artist": no_artist,
         "date_from": date_from, "date_to": date_to,
+        "include_hidden": include_hidden,
     }
     return filters, sort