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/accounts/serializers.py

@@ -2,7 +2,7 @@ from rest_framework import serializers
 
 from apps.billing.models import CreditAccount
 
-from .models import Team, TeamMember, User
+from .models import LoginSession, Team, TeamMember, User, UserPreference
 
 
 class UserSerializer(serializers.ModelSerializer):
@@ -12,6 +12,26 @@ class UserSerializer(serializers.ModelSerializer):
         read_only_fields = ["id", "status"]
 
 
+class UserPreferenceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = UserPreference
+        fields = ["notify", "two_factor_enabled", "creation_defaults", "display", "updated_at"]
+        read_only_fields = ["updated_at"]
+
+
+class LoginSessionSerializer(serializers.ModelSerializer):
+    is_current = serializers.SerializerMethodField()
+
+    class Meta:
+        model = LoginSession
+        fields = ["id", "user_agent", "ip_address", "last_seen_at", "created_at", "is_current"]
+        read_only_fields = fields
+
+    def get_is_current(self, obj) -> bool:
+        ctx = self.context or {}
+        return bool(obj.ip_address and obj.ip_address == ctx.get("current_ip") and obj.user_agent == ctx.get("current_ua"))
+
+
 class TeamSerializer(serializers.ModelSerializer):
     class Meta:
         model = Team