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/database.py

@@ -35,9 +35,48 @@ def init_db():
     from models import Feed, Article, FetchLog  # noqa
 
     Base.metadata.create_all(bind=engine)
+    _migrate(engine)
     init_fts5()
 
 
+def _migrate(engine):
+    """处理数据库增量迁移（添加新列）"""
+    import logging
+    logger = logging.getLogger(__name__)
+    conn = engine.raw_connection()
+    cursor = conn.cursor()
+
+    # 获取 feeds 表现有列
+    cursor.execute("PRAGMA table_info(feeds)")
+    existing = {row[1] for row in cursor.fetchall()}
+
+    migrations = [
+        ("feeds", "error_type", "VARCHAR(32) DEFAULT ''"),
+    ]
+    for table, column, col_type in migrations:
+        if column not in existing:
+            logger.info(f"迁移: ALTER TABLE {table} ADD COLUMN {column} {col_type}")
+            cursor.execute(f"ALTER TABLE {table} ADD COLUMN {column} {col_type}")
+
+    conn.commit()
+
+    # 对已有错误数据分类
+    from rss_fetcher import classify_error
+    cursor.execute("SELECT id, last_error FROM feeds WHERE last_error != '' AND (error_type IS NULL OR error_type = '')")
+    rows = cursor.fetchall()
+    for row in rows:
+        feed_id, error = row
+        etype = classify_error(error)
+        if etype:
+            cursor.execute("UPDATE feeds SET error_type = ? WHERE id = ?", (etype, feed_id))
+    if rows:
+        conn.commit()
+        logger.info(f"迁移: 已分类 {len(rows)} 条历史错误")
+
+    cursor.close()
+    conn.close()
+
+
 def init_fts5():
     """初始化 FTS5 全文搜索虚拟表"""
     conn = engine.raw_connection()