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/pkg/websearch/manager.go

@@ -200,13 +200,20 @@ func sortByStableRandomWeight(items []weighted) {
 	if len(items) <= 1 {
 		return
 	}
-	factors := make([]float64, len(items))
-	for i, item := range items {
-		factors[i] = float64(item.weight) * (0.5 + rand.Float64())
+	type entry struct {
+		item   weighted
+		factor float64
 	}
-	sort.Slice(items, func(i, j int) bool {
-		return factors[i] > factors[j]
+	entries := make([]entry, len(items))
+	for i, item := range items {
+		entries[i] = entry{item: item, factor: float64(item.weight) * (0.5 + rand.Float64())}
+	}
+	sort.Slice(entries, func(i, j int) bool {
+		return entries[i].factor > entries[j].factor
 	})
+	for i, e := range entries {
+		items[i] = e.item
+	}
 }
 
 func mergeWeightedResults(withQuota, withoutQuota []weighted, capacity int) []ProviderConfig {