fix(server): default timeline thread visibility to METADATA (#21669)

Orphaned messageChannelMessageAssociation rows (channel deleted in core,
association left behind when cleanup cron was down) made visibility
unresolvable, so formatThreads emitted null for the non-nullable
TimelineThread.visibility field and 500'd the whole timeline query.

Fail closed to METADATA (most restrictive existing tier) so a missing
channel hides subject/body instead of breaking the page.

/closes TWENTY-SERVER-FM6

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packages/twenty-server/src/engine/core-modules/messaging/utils/format-threads.util.ts

@@ -25,7 +25,9 @@ export const formatThreads = (
   return threads
     .filter((thread) => isDefined(threadParticipantsByThreadId[thread.id]))
     .map((thread) => {
-      const visibility = threadVisibilityByThreadId[thread.id];
+      const visibility =
+        threadVisibilityByThreadId[thread.id] ??
+        MessageChannelVisibility.METADATA;
 
       return {
         ...thread,