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/repository/email_cache.go

@@ -12,11 +12,11 @@ import (
 )
 
 const (
-	verifyCodeKeyPrefix            = "verify_code:"
-	notifyVerifyKeyPrefix          = "notify_verify:"
-	passwordResetKeyPrefix         = "password_reset:"
-	passwordResetSentAtKeyPrefix   = "password_reset_sent:"
-	notifyCodeUserRateKeyPrefix    = "notify_code_user_rate:"
+	verifyCodeKeyPrefix          = "verify_code:"
+	notifyVerifyKeyPrefix        = "notify_verify:"
+	passwordResetKeyPrefix       = "password_reset:"
+	passwordResetSentAtKeyPrefix = "password_reset_sent:"
+	notifyCodeUserRateKeyPrefix  = "notify_code_user_rate:"
 )
 
 // verifyCodeKey generates the Redis key for email verification code.