fix(dm): restore ownerId=NULL semantics for 1-on-1 DM channels

- Add migrateFixOneOnOneOwnerIds migration to NULL-out ownerId on all
  existing 1-on-1 DMs (those with exactly 2 members)
- Fix POST /api/dm to create 1-on-1 channels with ownerId=null instead
  of the creator's ID
- Guard POST /api/dm/:id/members: reject with 400 if channel has no
  owner (i.e. is a 1-on-1), directing callers to POST /api/dm/group
- Guard DELETE /api/dm/:id/members: replace member-count check with
  ownerId check; remove now-duplicate dmChannel query in that handler
- Add CreateGroupDmRequest type to shared types

packages/server/src/db/migrate.ts

@@ -607,6 +607,8 @@ export function runMigrations(db: Database.Database): void {
     console.error('Federation mutation log backfill failed (non-fatal):', err);
   }
 
+  migrateFixOneOnOneOwnerIds(db);
+
   console.log('Migrations complete.');
 }
 
@@ -1568,3 +1570,24 @@ function migrateDmChannelsFederatedId(db: Database.Database): void {
     console.error('migrateDmChannelsFederatedId: owner backfill failed (non-fatal):', err);
   }
 }
+
+/** Fix ownerId on 1-on-1 DMs: should be NULL, not the creator's ID */
+function migrateFixOneOnOneOwnerIds(db: Database.Database): void {
+  try {
+    const result = db.prepare(`
+      UPDATE dm_channels SET owner_id = NULL
+      WHERE id IN (
+        SELECT dm_channel_id FROM dm_members
+        GROUP BY dm_channel_id
+        HAVING COUNT(*) = 2
+      )
+      AND owner_id IS NOT NULL
+    `).run();
+
+    if (result.changes > 0) {
+      console.log(`[migrate] Fixed ownerId on ${result.changes} 1-on-1 DM channel(s) (set to NULL)`);
+    }
+  } catch (err) {
+    console.error('migrateFixOneOnOneOwnerIds failed (non-fatal):', err);
+  }
+}