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/projects/serializers.py

@@ -1,5 +1,7 @@
 from rest_framework import serializers
 
+from apps.assets.serializers import AssetFileSerializer
+
 from .models import (
     BaseAssetGroup,
     BgmTrack,
@@ -18,6 +20,16 @@ from .models import (
 )
 
 
+def _asset_preview_url(asset) -> str:
+    """资产主文件的可播放/可显示 URL(主图优先,其次首张),内嵌进各阶段序列化,
+    让前端缩略图不再依赖(分页 20 条的)团队 assets 列表解析——团队资产 >20 时新生成的图本会丢。"""
+    if asset is None:
+        return ""
+    files = list(asset.files.all())
+    primary = next((f for f in files if f.is_primary), files[0] if files else None)
+    return AssetFileSerializer().get_preview_url(primary) if primary else ""
+
+
 class ProjectStageSerializer(serializers.ModelSerializer):
     class Meta:
         model = ProjectStage
@@ -27,10 +39,11 @@ class ProjectStageSerializer(serializers.ModelSerializer):
 
 class VideoSegmentSerializer(serializers.ModelSerializer):
     adopted_asset = serializers.SerializerMethodField()
+    adopted_asset_url = serializers.SerializerMethodField()
 
     class Meta:
         model = VideoSegment
-        fields = ["id", "sort_order", "target_duration_seconds", "status", "error_message", "adopted_version", "adopted_asset"]
+        fields = ["id", "sort_order", "target_duration_seconds", "status", "error_message", "adopted_version", "adopted_asset", "adopted_asset_url"]
         read_only_fields = ["id", "sort_order", "target_duration_seconds", "status", "error_message", "adopted_version"]
 
     def get_adopted_asset(self, obj):
@@ -38,22 +51,39 @@ class VideoSegmentSerializer(serializers.ModelSerializer):
         version = obj.adopted_version
         return str(version.asset_id) if version and version.asset_id else None
 
+    def get_adopted_asset_url(self, obj) -> str:
+        version = obj.adopted_version
+        return _asset_preview_url(version.asset) if version is not None else ""
+
 
 class BaseAssetGroupSerializer(serializers.ModelSerializer):
     candidate_assets = serializers.PrimaryKeyRelatedField(many=True, read_only=True)
+    adopted_asset_url = serializers.SerializerMethodField()
+    candidate_asset_urls = serializers.SerializerMethodField()
 
     class Meta:
         model = BaseAssetGroup
-        fields = ["id", "kind", "prompt", "adopted_asset", "candidate_assets", "version", "metadata", "created_at"]
+        fields = ["id", "kind", "prompt", "adopted_asset", "adopted_asset_url", "candidate_assets", "candidate_asset_urls", "version", "metadata", "created_at"]
         read_only_fields = fields
 
+    def get_adopted_asset_url(self, obj) -> str:
+        return _asset_preview_url(obj.adopted_asset)
+
+    def get_candidate_asset_urls(self, obj) -> dict:
+        return {str(asset.id): _asset_preview_url(asset) for asset in obj.candidate_assets.all()}
+
 
 class StoryboardFrameSerializer(serializers.ModelSerializer):
+    asset_url = serializers.SerializerMethodField()
+
     class Meta:
         model = StoryboardFrame
-        fields = ["id", "script_segment", "asset", "sort_order", "prompt"]
+        fields = ["id", "script_segment", "asset", "asset_url", "sort_order", "prompt"]
         read_only_fields = fields
 
+    def get_asset_url(self, obj) -> str:
+        return _asset_preview_url(obj.asset)
+
 
 class StoryboardVersionSerializer(serializers.ModelSerializer):
     frames = StoryboardFrameSerializer(many=True, read_only=True)
@@ -72,10 +102,34 @@ class VideoSegmentVersionSerializer(serializers.ModelSerializer):
 
 
 class TimelineClipSerializer(serializers.ModelSerializer):
+    # 直接内嵌片段资产的可播放 URL + 是否视频,前端播放器无需再依赖(分页的)团队 assets 列表解析
+    asset_url = serializers.SerializerMethodField()
+    asset_is_video = serializers.SerializerMethodField()
+
     class Meta:
         model = TimelineClip
-        fields = ["id", "asset", "sort_order", "start_ms", "duration_ms", "trim_start_ms", "trim_end_ms"]
-        read_only_fields = ["id"]
+        fields = ["id", "asset", "asset_url", "asset_is_video", "sort_order", "start_ms", "duration_ms", "trim_start_ms", "trim_end_ms"]
+        read_only_fields = ["id", "asset_url", "asset_is_video"]
+
+    def _primary_file(self, obj):
+        asset = obj.asset
+        if asset is None:
+            return None
+        files = list(asset.files.all())
+        return next((f for f in files if f.is_primary), files[0] if files else None)
+
+    def get_asset_url(self, obj) -> str:
+        f = self._primary_file(obj)
+        return AssetFileSerializer().get_preview_url(f) if f else ""
+
+    def get_asset_is_video(self, obj) -> bool:
+        asset = obj.asset
+        if asset is None:
+            return False
+        if asset.asset_type == "video":
+            return True
+        f = self._primary_file(obj)
+        return bool(f and "video/" in (f.content_type or ""))
 
 
 class TimelineExportJobSerializer(serializers.ModelSerializer):
@@ -93,11 +147,20 @@ class SubtitleTrackSerializer(serializers.ModelSerializer):
 
 
 class BgmTrackSerializer(serializers.ModelSerializer):
+    asset_url = serializers.SerializerMethodField()
+    asset_name = serializers.SerializerMethodField()
+
     class Meta:
         model = BgmTrack
-        fields = ["id", "asset", "volume", "start_ms"]
+        fields = ["id", "asset", "asset_url", "asset_name", "volume", "start_ms"]
         read_only_fields = fields
 
+    def get_asset_url(self, obj) -> str:
+        return _asset_preview_url(obj.asset)
+
+    def get_asset_name(self, obj) -> str:
+        return obj.asset.name if obj.asset_id else ""
+
 
 class TimelineSerializer(serializers.ModelSerializer):
     clips = TimelineClipSerializer(many=True, read_only=True)