fix: websearch features_config cleanup and pricing rules validation

- Fix web_search_emulation toggle: explicitly write false for disabled
  platforms instead of leaving stale true from cloned features_config
- Extract validatePricingEntries from validateChannelConfig for reuse
- Validate account_stats_pricing_rules[].pricing in both Create and
  Update paths (negative prices, bad intervals, missing per_request price)

frontend/src/views/admin/ChannelsView.vue

@@ -1032,15 +1032,19 @@ function formToAPI(): { group_ids: number[], model_pricing: ChannelModelPricing[
   }
 
   // Collect web_search_emulation (only anthropic platform supports it)
+  // Always write the key so that disabling in the UI correctly sets platform to false,
+  // rather than leaving a stale true value from the cloned features_config.
   const wsEmulation: Record<string, boolean> = {}
   for (const section of form.platforms) {
     if (!section.enabled) continue
-    if (section.web_search_emulation && section.platform === 'anthropic') {
-      wsEmulation[section.platform] = true
+    if (section.platform === 'anthropic') {
+      wsEmulation[section.platform] = !!section.web_search_emulation
     }
   }
   if (Object.keys(wsEmulation).length > 0) {
     featuresConfig.web_search_emulation = wsEmulation
+  } else {
+    delete featuresConfig.web_search_emulation
   }
 
   return { group_ids, model_pricing, model_mapping, features_config: featuresConfig }