新增搜索结果缓存功能

- 新增 search_cache 数据库表存储搜索缓存
- 搜索前优先查询缓存，命中则直接返回
- 缓存默认有效期7天，可配置
- 前端支持'使用缓存'开关选项
- 显示缓存状态提示
- 支持自动清理过期缓存

models/database.py

@@ -121,6 +121,20 @@ class Database:
                 )
             ''')
             
+            # 搜索缓存表
+            cursor.execute('''
+                CREATE TABLE IF NOT EXISTS search_cache (
+                    id INTEGER PRIMARY KEY AUTOINCREMENT,
+                    keyword TEXT NOT NULL,
+                    engine TEXT DEFAULT 'bing_cn',
+                    results TEXT NOT NULL,
+                    result_count INTEGER,
+                    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+                    expires_at DATETIME,
+                    UNIQUE(keyword, engine)
+                )
+            ''')
+            
             conn.commit()
 
 # ========== 内容库操作 ==========
@@ -396,5 +410,52 @@ class Database:
             conn.commit()
             return cursor.rowcount
 
+# ========== 搜索缓存操作 ==========
+    def save_search_cache(self, keyword, engine, results, expire_days=7):
+        """保存搜索结果缓存"""
+        with self.get_connection() as conn:
+            cursor = conn.cursor()
+            cursor.execute('''
+                INSERT OR REPLACE INTO search_cache (keyword, engine, results, result_count, created_at, expires_at)
+                VALUES (?, ?, ?, ?, CURRENT_TIMESTAMP, datetime('now', '+' || ? || ' days'))
+            ''', (keyword, engine, json.dumps(results, ensure_ascii=False), len(results), expire_days))
+            conn.commit()
+            return cursor.lastrowid
+    
+    def get_search_cache(self, keyword, engine='bing_cn'):
+        """获取搜索结果缓存"""
+        with self.get_connection() as conn:
+            cursor = conn.cursor()
+            cursor.execute('''
+                SELECT results, result_count, created_at, expires_at 
+                FROM search_cache 
+                WHERE keyword = ? AND engine = ? AND expires_at > datetime('now')
+            ''', (keyword, engine))
+            row = cursor.fetchone()
+            if row:
+                return {
+                    'results': json.loads(row['results']),
+                    'count': row['result_count'],
+                    'cached_at': row['created_at'],
+                    'expires_at': row['expires_at']
+                }
+            return None
+    
+    def clear_expired_cache(self):
+        """清理过期缓存"""
+        with self.get_connection() as conn:
+            cursor = conn.cursor()
+            cursor.execute('DELETE FROM search_cache WHERE expires_at <= datetime("now")')
+            conn.commit()
+            return cursor.rowcount
+    
+    def clear_all_cache(self):
+        """清空所有缓存"""
+        with self.get_connection() as conn:
+            cursor = conn.cursor()
+            cursor.execute('DELETE FROM search_cache')
+            conn.commit()
+            return cursor.rowcount
+
 # 全局数据库实例
 db = Database()