fix: audit round-3 — proxy safety, intervals persistence, SMTP timeout, sort fix

- Skip websearch provider when ProxyID is set but proxy not found (prevent
  silent direct connection bypass)
- Fix sortByStableRandomWeight: pair factors with items so sort.Slice swap
  keeps weights aligned
- Allow empty platform in account_stats_pricing_rules (wildcard matching),
  only force anthropic default for main model_pricing
- Add channel_account_stats_pricing_intervals table and repo layer support
  for interval-based pricing in account stats rules
- calculateTokenStatsCost now uses interval pricing when available
- Replace smtp.SendMail/tls.Dial with net.Dialer timeout (10s dial, 20s IO)
  to prevent goroutine leak on SMTP hang
- Fix gofmt formatting issues
- Web Search label: black text with red warning hint

backend/internal/handler/admin/channel_handler.go

@@ -249,9 +249,6 @@ func pricingRequestToService(reqs []channelModelPricingRequest) []service.Channe
 			billingMode = service.BillingModeToken
 		}
 		platform := r.Platform
-		if platform == "" {
-			platform = service.PlatformAnthropic
-		}
 		intervals := make([]service.PricingInterval, 0, len(r.Intervals))
 		for _, iv := range r.Intervals {
 			intervals = append(intervals, service.PricingInterval{
@@ -349,6 +346,12 @@ func (h *ChannelHandler) Create(c *gin.Context) {
 	}
 
 	pricing := pricingRequestToService(req.ModelPricing)
+	// Main model_pricing requires a platform; default to anthropic for backward compatibility.
+	for i := range pricing {
+		if pricing[i].Platform == "" {
+			pricing[i].Platform = service.PlatformAnthropic
+		}
+	}
 
 	var statsRules []service.AccountStatsPricingRule
 	for i, r := range req.AccountStatsPricingRules {
@@ -415,6 +418,11 @@ func (h *ChannelHandler) Update(c *gin.Context) {
 	}
 	if req.ModelPricing != nil {
 		pricing := pricingRequestToService(*req.ModelPricing)
+		for i := range pricing {
+			if pricing[i].Platform == "" {
+				pricing[i].Platform = service.PlatformAnthropic
+			}
+		}
 		input.ModelPricing = &pricing
 	}
 	if req.AccountStatsPricingRules != nil {