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/models.py

@@ -1,5 +1,5 @@
 """SQLAlchemy 数据模型"""
-from datetime import datetime, timezone
+from datetime import datetime
 from sqlalchemy import Column, Integer, String, Text, Boolean, DateTime, ForeignKey
 from sqlalchemy.orm import relationship
 from database import Base
@@ -21,11 +21,12 @@ class Feed(Base):
     last_fetch_at = Column(DateTime, nullable=True)
     last_fetch_status = Column(String(20), default="")
     last_error = Column(Text, default="")
+    error_type = Column(String(32), default="")
     success_count = Column(Integer, default=0)
     fail_count = Column(Integer, default=0)
     article_count = Column(Integer, default=0)
 
-    created_at = Column(DateTime, default=lambda: datetime.now(timezone.utc))
+    created_at = Column(DateTime, default=datetime.utcnow)
 
     # 关联
     articles = relationship("Article", back_populates="feed", cascade="all, delete-orphan")
@@ -36,6 +37,7 @@ class Feed(Base):
         🟢 健康: 成功率 >= 90%, 最近7天有更新
         🟡 警告: 成功率 50%-90%, 或超过3天未更新
         🔴 异常: 成功率 < 50%, 或超过7天未更新
+        ⚪ 未知: 尚未进行过任何抓取
         """
         total = self.success_count + self.fail_count
         if total == 0:
@@ -44,7 +46,7 @@ class Feed(Base):
         success_rate = self.success_count / total
 
         if now is None:
-            now = datetime.now(timezone.utc)
+            now = datetime.utcnow()
 
         days_since_last_fetch = None
         if self.last_fetch_at:
@@ -71,7 +73,7 @@ class Article(Base):
     content = Column(Text, default="")
     summary = Column(Text, default="")
     is_read = Column(Boolean, default=False)
-    created_at = Column(DateTime, default=lambda: datetime.now(timezone.utc), index=True)
+    created_at = Column(DateTime, default=datetime.utcnow, index=True)
 
     # 关联
     feed = relationship("Feed", back_populates="articles")
@@ -87,7 +89,7 @@ class FetchLog(Base):
     articles_fetched = Column(Integer, default=0)
     error_message = Column(Text, default="")
     response_time_ms = Column(Integer, nullable=True)
-    created_at = Column(DateTime, default=lambda: datetime.now(timezone.utc), index=True)
+    created_at = Column(DateTime, default=datetime.utcnow, index=True)
 
     # 关联
     feed = relationship("Feed", back_populates="fetch_logs")