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/shared/src/types.ts

@@ -516,6 +516,10 @@ export interface AddDmMemberRequest {
   userId: string;
 }
 
+export interface CreateGroupDmRequest {
+  userIds: string[];
+}
+
 export interface CreateDmMessageRequest {
   content?: string;
   attachments?: string[];