feat(core/backend): pipeline continuity + threaded ffmpeg burn-in export + upload/save-timeline

Video pipeline (script→assets→storyboard→video→stitch):
- robust split_script_into_segments (4 non-empty scenes), scene-aware storyboard/video prompts
- link VideoSegment→ScriptSegment + storyboard-frame reference image (graceful text fallback)
- idempotent poll_video_segment (no double-charge on repeated polling)
- threaded export (no Celery worker needed) + poll-export endpoint
- run_export_job rewritten to filter_complex: per-clip trim, xfade transitions,
  subtitle burn-in (Pillow PNG overlay; this ffmpeg lacks libass), BGM mix
- upload-video-segment / upload-bgm / save-timeline endpoints
- serializers embed asset preview URLs (beat assets pagination); Pillow added to requirements

Also includes prior uncommitted backend work: account preferences/sessions,
billing trend, product/asset endpoints, accounts 0002 migration.

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

core/backend/apps/billing/urls.py

@@ -1,9 +1,10 @@
 from django.urls import path
 
-from .views import ledgers, recharge, summary
+from .views import ledgers, recharge, summary, trend
 
 urlpatterns = [
     path("summary/", summary, name="billing-summary"),
     path("ledgers/", ledgers, name="billing-ledgers"),
     path("recharge/", recharge, name="billing-recharge"),
+    path("trend/", trend, name="billing-trend"),
 ]