feat(core/backend): serve TOS presigned preview_url for asset files

AssetFile.preview_url was stored blank on upload, so all thumbnails fell back to placeholders.
Make preview_url a SerializerMethodField that signs a TOS GET URL from object_key on read
(falls back to stored value, or "" when TOS unconfigured / no key). Verified: presigned URL
for an existing object returns HTTP 200 image/png.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/backend/apps/assets/serializers.py

@@ -1,9 +1,23 @@
+from django.conf import settings
 from rest_framework import serializers
 
 from .models import Asset, AssetFile
+from .storage import TosStorage
+
+
+_tos_storage = None
+
+
+def _tos():
+    global _tos_storage
+    if _tos_storage is None:
+        _tos_storage = TosStorage()
+    return _tos_storage
 
 
 class AssetFileSerializer(serializers.ModelSerializer):
+    preview_url = serializers.SerializerMethodField()
+
     class Meta:
         model = AssetFile
         fields = [
@@ -18,7 +32,28 @@ class AssetFileSerializer(serializers.ModelSerializer):
             "preview_url",
             "is_primary",
         ]
-        read_only_fields = fields
+        read_only_fields = [
+            "id",
+            "object_key",
+            "bucket",
+            "content_type",
+            "size_bytes",
+            "width",
+            "height",
+            "duration_ms",
+            "is_primary",
+        ]
+
+    def get_preview_url(self, obj):
+        # 存储字段优先(如外部已写入绝对 URL);否则用 object_key 实时签发 TOS 预签名 GET URL
+        if obj.preview_url:
+            return obj.preview_url
+        if not obj.object_key or not settings.TOS.get("endpoint"):
+            return ""
+        try:
+            return _tos().presigned_get_url(object_key=obj.object_key)
+        except Exception:
+            return ""
 
 
 class AssetSerializer(serializers.ModelSerializer):