chore: retire zen-backed summarization

Disable the active Zen summarization flow because the unauthenticated/free Zen provider is no longer available and now returns usage-limit errors for this feature.

Keep /api/text/summarize as an API-compatible stub that returns local sanitized or distilled fallback text with summarized=false, rather than attempting external model calls.

Remove notification and voice playback summary behavior from runtime paths. Notification {last_message} now always uses normalized truncated text, and TTS playback ignores historical summarize request fields.

Hide the notification summary settings and voice summarize-before-playback controls while preserving legacy persisted settings for compatibility. Also disable Zen model startup validation and make Zen model list routes return empty results.

Update module documentation and tests to describe the retired provider behavior and the remaining compatibility stubs.

packages/web/server/index.js

@@ -249,9 +249,6 @@ const formatProjectLabel = (...args) => notificationTemplateRuntime.formatProjec
 const resolveNotificationTemplate = (...args) => notificationTemplateRuntime.resolveNotificationTemplate(...args);
 const shouldApplyResolvedTemplateMessage = (...args) => notificationTemplateRuntime.shouldApplyResolvedTemplateMessage(...args);
 const fetchFreeZenModels = (...args) => notificationTemplateRuntime.fetchFreeZenModels(...args);
-const resolveZenModel = (...args) => notificationTemplateRuntime.resolveZenModel(...args);
-const validateZenModelAtStartup = (...args) => notificationTemplateRuntime.validateZenModelAtStartup(...args);
-const summarizeText = (...args) => notificationTemplateRuntime.summarizeText(...args);
 const extractTextFromParts = (...args) => notificationTemplateRuntime.extractTextFromParts(...args);
 const extractLastMessageText = (...args) => notificationTemplateRuntime.extractLastMessageText(...args);
 const fetchLastAssistantMessageText = (...args) => notificationTemplateRuntime.fetchLastAssistantMessageText(...args);
@@ -668,8 +665,6 @@ notificationTemplateRuntime = createNotificationTemplateRuntime({
 const notificationTriggerRuntime = createNotificationTriggerRuntime({
   readSettingsFromDisk,
   prepareNotificationLastMessage,
-  summarizeText,
-  resolveZenModel,
   buildTemplateVariables,
   extractLastMessageText,
   fetchLastAssistantMessageText,
@@ -1081,9 +1076,6 @@ async function main(options = {}) {
 
   const sayTTSCapability = await detectSayTtsCapability(process);
 
-  // Startup model validation is best-effort and runs in background.
-  void validateZenModelAtStartup();
-
   const app = express();
   const serverStartedAt = new Date().toISOString();
   app.set('trust proxy', true);
@@ -1138,7 +1130,6 @@ async function main(options = {}) {
     tunnelAuthController,
     readSettingsFromDiskMigrated,
     normalizeTunnelSessionTtlMs,
-    resolveZenModel,
     sayTTSCapability,
     ensurePushInitialized,
     ensureGlobalWatcherStarted,