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/server/http.go

@@ -4,6 +4,7 @@ package server
 import (
 	"context"
 	"log"
+	"log/slog"
 	"net/http"
 	"time"
 
@@ -82,6 +83,11 @@ func ProvideRouter(
 				pc.ProxyID = *p.ProxyID
 				if u, ok := proxyURLs[*p.ProxyID]; ok {
 					pc.ProxyURL = u
+				} else {
+					// Proxy configured but not found — skip this provider to prevent direct connection.
+					slog.Warn("websearch: proxy not found for provider, skipping",
+						"provider", p.Type, "proxy_id", *p.ProxyID)
+					continue
 				}
 			}
 			configs = append(configs, pc)