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);
+  }
+}

packages/server/src/routes/dm.ts

@@ -444,7 +444,7 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
     db.transaction((tx) => {
       tx.insert(schema.dmChannels).values({
         id: dmChannelId,
-        ownerId: request.userId,
+        ownerId: null,
         createdAt: now,
       }).run();
 
@@ -466,7 +466,7 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
 
     const result: DmChannel = {
       id: dmChannelId,
-      ownerId: request.userId,
+      ownerId: null,
       createdAt: now,
       members,
       lastMessage: null,
@@ -537,12 +537,16 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
       return reply.code(403).send({ error: 'You are not a member of this DM channel', statusCode: 403 });
     }
 
-    // Enforce DM channel ownership: only the owner can add members (for new-style group DMs)
+    // Fetch channel and enforce type + ownership constraints
     let dmChannel = db.select().from(schema.dmChannels).where(and(eq(schema.dmChannels.id, id), isNull(schema.dmChannels.deletedAt))).get();
     if (!dmChannel) {
       return reply.code(404).send({ error: 'DM channel not found', statusCode: 404 });
     }
-    if (dmChannel.ownerId && dmChannel.ownerId !== request.userId) {
+    // 1-on-1 DMs (ownerId=NULL) are immutable — cannot add members
+    if (!dmChannel.ownerId) {
+      return reply.code(400).send({ error: 'Cannot add members to a 1-on-1 DM. Use POST /api/dm/group to create a group.', statusCode: 400 });
+    }
+    if (dmChannel.ownerId !== request.userId) {
       return reply.code(403).send({ error: 'Only the group owner can add members', statusCode: 403 });
     }
 
@@ -759,14 +763,13 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
       return reply.code(403).send({ error: 'You are not a member of this DM channel', statusCode: 403 });
     }
 
-    // Count members — can't leave a 1-on-1
-    const memberRows = db.select()
-      .from(schema.dmMembers)
-      .where(eq(schema.dmMembers.dmChannelId, id))
-      .all();
-
-    if (memberRows.length <= 2) {
-      return reply.code(400).send({ error: 'Cannot leave a 1-on-1 DM. Use close instead.', statusCode: 400 });
+    // Fetch channel — 1-on-1 DMs (ownerId=NULL) cannot be left, only closed
+    const dmChannel = db.select().from(schema.dmChannels).where(and(eq(schema.dmChannels.id, id), isNull(schema.dmChannels.deletedAt))).get();
+    if (!dmChannel) {
+      return reply.code(404).send({ error: 'DM channel not found', statusCode: 404 });
+    }
+    if (!dmChannel.ownerId) {
+      return reply.code(400).send({ error: 'Cannot leave a 1-on-1 DM. Use DELETE /api/dm/:id to close it.', statusCode: 400 });
     }
 
     // If user is in this DM's VoiceRoom, leave it first
@@ -794,9 +797,6 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
       connectionManager.clearVoiceUserStatus(request.userId);
     }
 
-    // Check DM channel ownership before leaving
-    const dmChannel = db.select().from(schema.dmChannels).where(and(eq(schema.dmChannels.id, id), isNull(schema.dmChannels.deletedAt))).get();
-
     // Compute federation targets BEFORE member deletion so the leaving user's peer is included
     let fedTargetOrigins: string[] | undefined;
     let leavingUser: typeof schema.users.$inferSelect | undefined;