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/web/embed_off.go

@@ -1,20 +1,20 @@
-//go:build !embed
-
-package web
-
-import (
-	"net/http"
-
-	"github.com/gin-gonic/gin"
-)
-
-func ServeEmbeddedFrontend() gin.HandlerFunc {
-	return func(c *gin.Context) {
-		c.String(http.StatusNotFound, "Frontend not embedded. Build with -tags embed to include frontend.")
-		c.Abort()
-	}
-}
-
-func HasEmbeddedFrontend() bool {
-	return false
-}
+//go:build !embed
+
+package web
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+func ServeEmbeddedFrontend() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.String(http.StatusNotFound, "Frontend not embedded. Build with -tags embed to include frontend.")
+		c.Abort()
+	}
+}
+
+func HasEmbeddedFrontend() bool {
+	return false
+}