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/service/gemini_oauth.go

@@ -1,13 +1,13 @@
-package service
-
-import (
-	"context"
-
-	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
-)
-
-// GeminiOAuthClient performs Google OAuth token exchange/refresh for Gemini integration.
-type GeminiOAuthClient interface {
-	ExchangeCode(ctx context.Context, oauthType, code, codeVerifier, redirectURI, proxyURL string) (*geminicli.TokenResponse, error)
-	RefreshToken(ctx context.Context, oauthType, refreshToken, proxyURL string) (*geminicli.TokenResponse, error)
-}
+package service
+
+import (
+	"context"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
+)
+
+// GeminiOAuthClient performs Google OAuth token exchange/refresh for Gemini integration.
+type GeminiOAuthClient interface {
+	ExchangeCode(ctx context.Context, oauthType, code, codeVerifier, redirectURI, proxyURL string) (*geminicli.TokenResponse, error)
+	RefreshToken(ctx context.Context, oauthType, refreshToken, proxyURL string) (*geminicli.TokenResponse, error)
+}