feat: websearch quota enhancements and balance notify hint

- QuotaLimit changed to *int64 (null=unlimited, >0=limited)
- Add reset-usage endpoint (POST /admin/settings/web-search-emulation/reset-usage)
- Show quota usage in header always (collapsed and expanded)
- Add reset quota button in expanded provider view
- Quota input: empty=unlimited with ∞ placeholder, must be >0 if set
- Add email verification hint on balance notify card

backend/internal/server/http.go

@@ -73,7 +73,7 @@ func ProvideRouter(
 			pc := websearch.ProviderConfig{
 				Type:       p.Type,
 				APIKey:     p.APIKey,
-				QuotaLimit: p.QuotaLimit,
+				QuotaLimit: derefInt64(p.QuotaLimit),
 				ExpiresAt:  p.ExpiresAt,
 			}
 			if p.SubscribedAt != nil {
@@ -141,3 +141,10 @@ func ProvideHTTPServer(cfg *config.Config, router *gin.Engine) *http.Server {
 		// 不设置 ReadTimeout，因为大请求体可能需要较长时间读取
 	}
 }
+
+func derefInt64(p *int64) int64 {
+	if p == nil {
+		return 0
+	}
+	return *p
+}