backlot phase 2: board UI (library, live board, filmstrip, script modal, drawers)

- vanilla ESM frontend on the mockup design system (no build step):
  library contact-sheet with mini rails + live badges; project board with
  slate header, cost meter, clickable stage rail, stage drawers (artifact
  view, review findings, gate-skipped chip, versions), screenplay card +
  full-script modal, decisions + activity rails, storyboard filmstrip
  (duration-width cards, shimmer/spec/missing/text-card states, takes,
  narration + waveform playback), render player with versions, degraded
  found-media view
- /thumb endpoint: cached downscaled JPEGs (Pillow) + ffmpeg poster-frame
  extraction for videos — the library was loading 73 full-res PNGs
- library summaries cached, invalidated by the watcher
- asset path resolution tolerates project-relative, repo-relative and
  absolute manifest paths (real-world variance found in why-do-we-dream)
- ?static=1 disables SSE (screenshots/static export)
- verified in browser against signal-from-tomorrow, why-do-we-dream,
  done-beats-perfect and the 73-project library

.gitignore

@@ -94,3 +94,6 @@ remotion-composer/public/demo-props/caption-burn-*
 
 venv/
 .venv/
+
+# Backlot local cache (thumbnails)
+.backlot/