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/handler/admin/setting_handler.go

@@ -1962,7 +1962,28 @@ func (h *SettingHandler) UpdateWebSearchEmulationConfig(c *gin.Context) {
 		response.ErrorFrom(c, err)
 		return
 	}
-	response.Success(c, service.SanitizeWebSearchConfig(c.Request.Context(), updated))
+	response.Success(c, service.PopulateWebSearchUsage(c.Request.Context(), updated))
+}
+
+// ResetWebSearchUsage 重置指定 provider 的配额用量
+// POST /api/v1/admin/settings/web-search-emulation/reset-usage
+func (h *SettingHandler) ResetWebSearchUsage(c *gin.Context) {
+	var req struct {
+		ProviderType string `json:"provider_type"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "Invalid request: "+err.Error())
+		return
+	}
+	if req.ProviderType == "" {
+		response.BadRequest(c, "provider_type is required")
+		return
+	}
+	if err := service.ResetWebSearchUsage(c.Request.Context(), req.ProviderType); err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+	response.Success(c, nil)
 }
 
 // TestWebSearchEmulation 测试 Web Search 搜索