feat: 重构数据库配置并支持多数据库类型

- 将数据库配置从硬编码改为从外部文件加载
- 支持MySQL 5.7+和SQLite两种数据库类型
- 改进安装向导流程，增加数据库类型选择
- 更新README文档说明数据库要求
- 替换CDN链接为国内镜像源
- 增加安装完成后的安全提醒

重构数据库类以支持更灵活的配置方式，包括端口设置和MySQL 5.7兼容性处理。安装向导现在可以正确配置SQLite或MySQL数据库，并生成相应的配置文件。同时优化了前端资源加载速度。

config/database.php

@@ -5,12 +5,25 @@
  */
 
 class Database {
-    private $host = 'localhost';
-    private $db_name = 'submission_system';
-    private $username = 'root';
-    private $password = '';
+    private $host;
+    private $port;
+    private $db_name;
+    private $username;
+    private $password;
+    private $use_sqlite;
+    private $is_mysql_57;
     private $conn;
-    private $use_sqlite = false; // 设置为true使用SQLite
+    
+    public function __construct() {
+        $db_config = @include __DIR__ . '/db_config.php';
+        $this->host = $db_config['host'] ?? 'localhost';
+        $this->port = $db_config['port'] ?? '3306';
+        $this->db_name = $db_config['db_name'] ?? 'submission_system';
+        $this->username = $db_config['username'] ?? 'root';
+        $this->password = $db_config['password'] ?? '';
+        $this->use_sqlite = $db_config['use_sqlite'] ?? false;
+        $this->is_mysql_57 = $db_config['is_mysql_57'] ?? false;
+    }
     
     public function getConnection() {
         $this->conn = null;
@@ -18,22 +31,20 @@ class Database {
         try {
             if ($this->use_sqlite) {
                 // SQLite配置
-                $this->conn = new PDO('sqlite:' . __DIR__ . '/../data/database.sqlite');
+                $this->conn = new PDO('sqlite:' . __DIR__ . '/../data/submission.db');
             } else {
                 // MySQL配置
-                $this->conn = new PDO(
-                    "mysql:host=" . $this->host . ";dbname=" . $this->db_name . ";charset=utf8mb4",
-                    $this->username,
-                    $this->password
-                );
+                $dsn = "mysql:host=" . $this->host . ";port=" . $this->port . ";dbname=" . $this->db_name . ";charset=utf8mb4";
+                $this->conn = new PDO($dsn, $this->username, $this->password, [
+                    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
+                    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
+                    PDO::ATTR_EMULATE_PREPARES => false,
+                ]);
             }
-            $this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
-            $this->conn->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
-        } catch(PDOException $exception) {
-            echo "连接失败: " . $exception->getMessage();
+            return $this->conn;
+        } catch(PDOException $e) {
+            throw new Exception("数据库连接失败: " . $e->getMessage());
         }
-        
-        return $this->conn;
     }
     
     public function initDatabase() {
@@ -143,5 +154,9 @@ class Database {
             return false;
         }
     }
+
+    public function isMySQL57() {
+        return $this->is_mysql_57;
+    }
 }
 ?>