fix(db): 处理MySQL连接的只读副本错误

在连接MySQL时，增加对只读副本可能返回的错误（8001和polar slave）的处理逻辑，确保连接在出现此类错误时仍然可用。

server/internal/db/mysql.go

@@ -2,6 +2,7 @@ package db
 
 import (
     "database/sql"
+    "strings"
 
     _ "github.com/go-sql-driver/mysql"
 )
@@ -12,6 +13,16 @@ func ConnectMySQL(dsn string) (*sql.DB, error) {
         return nil, err
     }
     if err := db.Ping(); err != nil {
+        // Error 8001: Not allowed to execute statement that may change data on polar slave
+        // This error can occur on read-only replicas but the connection may still be usable
+        // Try a simple SELECT to verify the connection is actually working
+        if strings.Contains(err.Error(), "8001") || strings.Contains(err.Error(), "polar slave") {
+            var dummy int
+            if testErr := db.QueryRow("SELECT 1").Scan(&dummy); testErr == nil {
+                // Connection is actually working, ignore the ping error
+                return db, nil
+            }
+        }
         return nil, err
     }
     return db, nil