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/routes/admin.go

@@ -411,6 +411,7 @@ func registerSettingsRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		adminSettings.GET("/web-search-emulation", h.Admin.Setting.GetWebSearchEmulationConfig)
 		adminSettings.PUT("/web-search-emulation", h.Admin.Setting.UpdateWebSearchEmulationConfig)
 		adminSettings.POST("/web-search-emulation/test", h.Admin.Setting.TestWebSearchEmulation)
+		adminSettings.POST("/web-search-emulation/reset-usage", h.Admin.Setting.ResetWebSearchUsage)
 	}
 }