fix: repair corrupted group DMs, prevent ownerId nulling, show empty groups

Three fixes for group DM data integrity and display:

1. processOwnershipTransferEvent: use resolveOrCreateReplicatedUser instead
   of resolveLocalUser to guarantee a valid ownerId. The previous ?? null
   fallback converted group DMs into 1-on-1s when resolution failed.

2. Self-healing migration: detect group DMs with UUID-format federated_id
   but NULL owner_id (corrupted by the old fallback) and restore owner from
   the first remaining member. Found and repaired 7 across both instances.

3. Sidebar: group DMs with 0 other members (last person standing) now show
   as "Empty Group" instead of being hidden. 1-on-1 DMs with 0 others are
   still correctly filtered out.

packages/server/src/db/migrate.ts

@@ -635,6 +635,37 @@ export function runMigrations(db: Database.Database): void {
     db.exec(`ALTER TABLE federation_peers ADD COLUMN remote_max_upload_size INTEGER`);
   } catch { /* column already exists */ }
 
+  // ─── Data integrity: repair group DMs with nulled-out owner_id ───────────
+  // A bug in processOwnershipTransferEvent (fixed in cd7aff0) could set
+  // owner_id to NULL when resolveLocalUser failed, converting a group DM into
+  // a 1-on-1-looking channel. Detect these by finding dm_channels with a
+  // UUID-format federated_id (group DMs) but NULL owner_id, and restore the
+  // owner from the first remaining member.
+  const corruptedGroups = db.prepare(`
+    SELECT c.id, c.federated_id
+    FROM dm_channels c
+    WHERE c.owner_id IS NULL
+      AND c.federated_id IS NOT NULL
+      AND c.deleted_at IS NULL
+      AND length(c.federated_id) = 36
+      AND c.federated_id LIKE '________-____-____-____-____________'
+  `).all() as Array<{ id: string; federated_id: string }>;
+
+  for (const ch of corruptedGroups) {
+    const firstMember = db.prepare(
+      `SELECT user_id FROM dm_members WHERE dm_channel_id = ? LIMIT 1`
+    ).get(ch.id) as { user_id: string } | undefined;
+
+    if (firstMember) {
+      db.prepare(`UPDATE dm_channels SET owner_id = ? WHERE id = ?`).run(firstMember.user_id, ch.id);
+      console.log(`[migration] Repaired group DM ${ch.id}: restored owner_id to ${firstMember.user_id}`);
+    }
+  }
+
+  if (corruptedGroups.length > 0) {
+    console.log(`[migration] Repaired ${corruptedGroups.length} corrupted group DM(s).`);
+  }
+
   console.log('Migrations complete.');
 }
 

packages/server/src/routes/federation.ts

@@ -1967,12 +1967,18 @@ function processOwnershipTransferEvent(
     return;
   }
 
-  // Resolve new owner to local user (for local ownerId)
-  const newOwnerLocal = resolveLocalUser(event.ownership.newOwner.homeUserId, db);
+  // Resolve new owner to local user — use resolveOrCreateReplicatedUser to
+  // guarantee we always get a valid user ID. Never fall back to null, as that
+  // would convert the group DM into a 1-on-1 and destroy its type identity.
+  const newOwnerLocal = resolveOrCreateReplicatedUser(
+    event.ownership.newOwner.homeUserId,
+    event.ownership.newOwner.homeInstance,
+    db,
+  );
 
   db.update(schema.dmChannels)
     .set({
-      ownerId: newOwnerLocal?.id ?? null,
+      ownerId: newOwnerLocal.id,
       ownerHomeUserId: event.ownership.newOwner.homeUserId,
       ownerHomeInstance: event.ownership.newOwner.homeInstance,
     })
@@ -1982,7 +1988,7 @@ function processOwnershipTransferEvent(
   connectionManager.sendToDmMembers(channel.id, {
     type: 'dm_owner_updated',
     dmChannelId: channel.id,
-    newOwnerId: newOwnerLocal?.id ?? event.ownership.newOwner.homeUserId,
+    newOwnerId: newOwnerLocal.id,
   });
 
   const prevOwnerLocal = event.ownership.previousOwner
@@ -1998,8 +2004,8 @@ function processOwnershipTransferEvent(
     userId: prevOwnerId,
     content: JSON.stringify({
       event: 'owner_changed',
-      newOwnerId: newOwnerLocal?.id ?? event.ownership.newOwner.homeUserId,
-      newOwnerDisplayName: newOwnerLocal?.displayName ?? newOwnerBaseName,
+      newOwnerId: newOwnerLocal.id,
+      newOwnerDisplayName: newOwnerLocal.displayName ?? newOwnerBaseName,
     }),
     type: 'system',
     createdAt: Date.now(),