feat: Introduce MediaAssetVariant for better asset management

- Added MediaAssetVariant enum with values 'main' and 'preview'.
- Updated media asset service logic to utilize MediaAssetVariant for variant handling.
- Refactored database models and queries to include variant and source_asset_id fields.
- Enhanced validation for asset variants in upload and processing functions.
- Updated Swagger documentation to reflect new variant structure and descriptions.
- Implemented necessary database migrations to support the new variant constraints.

backend/database/migrations/20251222174000_media_assets_variant.sql

@@ -9,9 +9,19 @@ SET variant = 'main'
 WHERE variant IS NULL OR variant = '';
 
 -- 约束：只允许 main/preview
-ALTER TABLE media_assets
-    ADD CONSTRAINT IF NOT EXISTS ck_media_assets_variant
-        CHECK (variant IN ('main', 'preview'));
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1
+        FROM pg_constraint
+        WHERE conname = 'ck_media_assets_variant'
+    ) THEN
+        ALTER TABLE media_assets
+            ADD CONSTRAINT ck_media_assets_variant
+                CHECK (variant IN ('main', 'preview'));
+    END IF;
+END
+$$;
 
 COMMENT ON COLUMN media_assets.variant IS '产物类型：main/preview；用于强制试看资源必须绑定独立产物，避免用正片绕过';
 
@@ -24,4 +34,3 @@ DROP INDEX IF EXISTS ix_media_assets_tenant_variant;
 ALTER TABLE media_assets DROP CONSTRAINT IF EXISTS ck_media_assets_variant;
 ALTER TABLE media_assets DROP COLUMN IF EXISTS variant;
 -- +goose StatementEnd
-