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/main.py

@@ -3,7 +3,6 @@ import os
 from contextlib import asynccontextmanager
 from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
-from fastapi.responses import FileResponse
 from starlette.middleware.cors import CORSMiddleware
 from database import init_db, SessionLocal
 from scheduler import init_feed_jobs, stop_scheduler
@@ -61,25 +60,7 @@ def health_check():
     return {"status": "ok", "service": "rssKeeper"}
 
 
-# 静态文件服务（前端构建产物）
+# 静态文件服务（前端构建产物）— 必须放在最后，API 路由优先匹配
 static_dir = os.path.join(config.BASE_DIR, "static")
 if os.path.exists(static_dir):
-    app.mount("/static", StaticFiles(directory=static_dir), name="static")
-
-    # API 路径白名单 — 这些路径不应被 SPA 兜底
-    _API_PATHS = {
-        "api", "docs", "openapi.json", "redoc",
-    }
-
-    @app.get("/{full_path:path}")
-    async def serve_spa(full_path: str):
-        """Vue SPA 路由回退"""
-        # API/文档路由不走 SPA 兜底
-        first_seg = full_path.split("/")[0] if full_path else ""
-        if first_seg in _API_PATHS:
-            return {"detail": "Not found"}
-
-        index_path = os.path.join(static_dir, "index.html")
-        if os.path.exists(index_path):
-            return FileResponse(index_path)
-        return {"detail": "Frontend not built"}
+    app.mount("/", StaticFiles(directory=static_dir, html=True), name="static")