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/web/src/components/layout/ChannelSidebar.tsx

@@ -489,14 +489,16 @@ export function ChannelSidebar() {
           <div className="space-y-[2px]">
             {dmChannels.map((dm) => {
               const otherMembers = dm.members.filter(m => !isSelf(m, user));
-              if (otherMembers.length === 0) return null;
               const isGroup = !!dm.ownerId;
+              if (otherMembers.length === 0 && !isGroup) return null;
               const isDmUnread = unreadChannels.has(dm.id) && currentChannelId !== dm.id;
 
               const firstOtherDm = isGroup ? null : otherMembers[0];
               const { baseName: dmBaseName, domain: dmDomain } = parseFederatedUsername(firstOtherDm?.username ?? '');
               const dmDisplayName = isGroup
-                ? otherMembers.map(m => m.displayName ?? parseFederatedUsername(m.username).baseName).join(', ')
+                ? (otherMembers.length > 0
+                  ? otherMembers.map(m => m.displayName ?? parseFederatedUsername(m.username).baseName).join(', ')
+                  : 'Empty Group')
                 : firstOtherDm?.displayName ?? dmBaseName;
 
               const dmItem = (