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

frontend/src/views/admin/ChannelsView.vue

@@ -328,10 +328,10 @@
             <div v-if="section.platform === 'anthropic' && webSearchGlobalEnabled" class="border-t border-gray-200 pt-3 dark:border-dark-600">
               <div class="flex items-center justify-between">
                 <div>
-                  <label class="text-xs font-medium text-orange-600 dark:text-orange-400">
+                  <label class="text-xs font-medium text-gray-700 dark:text-gray-300">
                     {{ t('admin.channels.form.webSearchEmulation') }}
                   </label>
-                  <p class="mt-0.5 text-[11px] text-amber-500 dark:text-amber-400">
+                  <p class="mt-0.5 text-[11px] text-red-500 dark:text-red-400">
                     {{ t('admin.channels.form.webSearchEmulationHint') }}
                   </p>
                 </div>