fix(scenes): JAV sites showed "No videos" when browsed directly

The JAV vertical gate excluded JAV origins whenever jav=false, including when the user
browsed a specific JAV site (Sites -> javflix/vjav/supjav -> SiteScenes passes
origin=... without jav=true), so those pages showed "No videos" (3 reports 07-19).

Only apply the exclusion to the main feed (origin is None). Browsing a specific origin
shows it regardless; the JAV tab (jav=true) still shows only JAV. Verified: javflix/vjav/
supjav now return scenes via origin filter, main feed still gates JAV out.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/scenes.py

@@ -349,10 +349,12 @@ def list_scenes(
         )
 
     # JAV vertical gate: domyślnie JAV wykluczone z głównego feedu; ?jav=true → TYLKO JAV.
-    # (origin=... diagnostyka nadal podlega bramie — do JAV użyj jav=true.)
+    # ALE gdy user browse'uje KONKRETNY origin (Sites → javflix/vjav/supjav → SiteScenes),
+    # bramy nie stosujemy — jawnie chce ten origin, więc pokazujemy (regresja "No videos"
+    # na stronach JAV, bug-report 2026-07-19). Brama dotyczy tylko głównego feedu (origin=None).
     if jav:
         base = base.where(_jav_source_exists())
-    else:
+    elif origin is None:
         base = base.where(~_jav_source_exists())
 
     # Blacklisty device (performer/studio/tag) — globalne wykluczenia, współdzielone z