feat: 深色主题UI、错误分类、批量抓取、健康度筛选

- 修复 datetime 时区不一致导致所有API 500错误的问题
- Feeds/Dashboard 页面改为深色表格主题，高对比度文字
- 添加错误类型自动分类（URL失效/被拒绝/超时/DNS失败/SSL错误等12种）
- 新增"下次抓取时间"列，从APScheduler获取
- 新增健康度筛选下拉，修复分页后过滤失效的bug
- "全部抓取"改为同步并发执行，基于当前筛选条件获取所有匹配源
- 新增数据库自动迁移机制，处理增量列变更

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/health_checker.py

@@ -1,5 +1,5 @@
 """RSS 源健康度检测"""
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timedelta
 from typing import List, Dict
 from sqlalchemy import func
 from sqlalchemy.orm import Session
@@ -10,7 +10,7 @@ def get_feed_health(db: Session, feed_id: int = None) -> List[Dict]:
     """获取 RSS 源健康度信息
     返回每个源的健康状态详情
     """
-    now = datetime.now(timezone.utc)
+    now = datetime.utcnow()
     query = db.query(Feed)
     if feed_id:
         query = query.filter(Feed.id == feed_id)
@@ -84,7 +84,7 @@ def get_overall_stats(db: Session) -> Dict:
     # 健康源统计
     feeds = db.query(Feed).all()
     healthy = warning = unhealthy = 0
-    now = datetime.now(timezone.utc)
+    now = datetime.utcnow()
     for feed in feeds:
         status = feed.health_status(now=now)
         if status == "healthy":