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/service/account_stats_pricing.go

@@ -195,18 +195,33 @@ func calculatePerRequestStatsCost(pricing *ChannelModelPricing, requestCount int
 }
 
 // calculateTokenStatsCost Token 计费。
+// If the pricing has intervals, find the matching interval by total token count
+// and use its prices instead of the flat pricing fields.
 func calculateTokenStatsCost(pricing *ChannelModelPricing, tokens UsageTokens) *float64 {
-	deref := func(p *float64) float64 {
-		if p == nil {
+	p := pricing
+	if len(pricing.Intervals) > 0 {
+		totalTokens := tokens.InputTokens + tokens.OutputTokens + tokens.CacheCreationTokens + tokens.CacheReadTokens
+		if iv := FindMatchingInterval(pricing.Intervals, totalTokens); iv != nil {
+			p = &ChannelModelPricing{
+				InputPrice:      iv.InputPrice,
+				OutputPrice:     iv.OutputPrice,
+				CacheWritePrice: iv.CacheWritePrice,
+				CacheReadPrice:  iv.CacheReadPrice,
+				PerRequestPrice: iv.PerRequestPrice,
+			}
+		}
+	}
+	deref := func(ptr *float64) float64 {
+		if ptr == nil {
 			return 0
 		}
-		return *p
+		return *ptr
 	}
-	cost := float64(tokens.InputTokens)*deref(pricing.InputPrice) +
-		float64(tokens.OutputTokens)*deref(pricing.OutputPrice) +
-		float64(tokens.CacheCreationTokens)*deref(pricing.CacheWritePrice) +
-		float64(tokens.CacheReadTokens)*deref(pricing.CacheReadPrice) +
-		float64(tokens.ImageOutputTokens)*deref(pricing.ImageOutputPrice)
+	cost := float64(tokens.InputTokens)*deref(p.InputPrice) +
+		float64(tokens.OutputTokens)*deref(p.OutputPrice) +
+		float64(tokens.CacheCreationTokens)*deref(p.CacheWritePrice) +
+		float64(tokens.CacheReadTokens)*deref(p.CacheReadPrice) +
+		float64(tokens.ImageOutputTokens)*deref(p.ImageOutputPrice)
 	if cost <= 0 {
 		return nil
 	}