refactor: 统一 sourceId 格式规范

规范: <source_type>_<user_id>_<timestamp_ms>

前端 4 处（projectId → userId）:
- VideoGeneration: video_${userId}_${Date.now()}
- SubtitleBurning: subtitle_burn_${userId}_${Date.now()}
- VideoCompose: compose_${userId}_${Date.now()}
- CoverDesign: cover_design_${userId}_${Date.now()}

后端 4 处（浮点秒 → 毫秒整数）:
- script.py polish: polish_${userId}_${int(time.time()*1000)}
- script.py title: title_${userId}_${int(time.time()*1000)}
- voice.py TTS: tts_${userId}_${int(time.time()*1000)}
- voice.py voice_clone: voice_clone_${userId}_${ts}_${voice_id}
  （原裸传第三方 voice_id，现包装为规范格式）

Refs: P2-1

python-api/app/api/v1/script.py

@@ -9,6 +9,7 @@ from __future__ import annotations
 
 import asyncio
 import logging
+import time
 
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -87,7 +88,7 @@ async def polish_content(
             user_id=current_user.id,
             points=points,
             source_type="polish",
-            source_id=f"polish_{current_user.id}_{asyncio.get_event_loop().time()}",
+            source_id=f"polish_{current_user.id}_{int(time.time() * 1000)}",
             description="【文案润色】",
         )
         await db.commit()
@@ -188,7 +189,7 @@ async def generate_title(
             user_id=current_user.id,
             points=points,
             source_type="title",
-            source_id=f"title_{current_user.id}_{asyncio.get_event_loop().time()}",
+            source_id=f"title_{current_user.id}_{int(time.time() * 1000)}",
             description="【标题生成】",
         )
         await db.commit()