feat: 品牌重命名 Sub2API -> TianShuAPI

- 前端: 所有界面显示、i18n 文本、组件中的品牌名称
- 后端: 服务层、设置默认值、邮件模板、安装向导
- 数据库: 迁移脚本注释
- 保持功能完全一致，仅更改品牌名称

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/internal/server/middleware/request_body_limit.go

@@ -1,15 +1,15 @@
-package middleware
-
-import (
-	"net/http"
-
-	"github.com/gin-gonic/gin"
-)
-
-// RequestBodyLimit 使用 MaxBytesReader 限制请求体大小。
-func RequestBodyLimit(maxBytes int64) gin.HandlerFunc {
-	return func(c *gin.Context) {
-		c.Request.Body = http.MaxBytesReader(c.Writer, c.Request.Body, maxBytes)
-		c.Next()
-	}
-}
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// RequestBodyLimit 使用 MaxBytesReader 限制请求体大小。
+func RequestBodyLimit(maxBytes int64) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Request.Body = http.MaxBytesReader(c.Writer, c.Request.Body, maxBytes)
+		c.Next()
+	}
+}