fix new user signup (tldraw#7093)

ayyy I forgot to add the groups_backend flag for new users in tldraw#7058

which means that people who have been signing up since then have still
been using the old data model. it's no biggie, just requires slight
adjustment to the migration script.

### Change type

- [ ] `bugfix`
- [ ] `improvement`
- [ ] `feature`
- [ ] `api`
- [x] `other`

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Ensure new users get `groups_backend` flag and update the migration to
use `ON CONFLICT DO NOTHING` for safe re-runs.
> 
> - **Backend**
> - Set `user.flags` to `groups_backend` on user creation in
`apps/dotcom/sync-worker/src/TLUserDurableObject.ts`.
> - **Database Migration**
(`apps/dotcom/zero-cache/migrations/023_groups.sql`)
> - Add `ON CONFLICT DO NOTHING` to inserts into `group`, `group_user`,
and `group_file` in `migrate_user_to_groups` to make it idempotent.
> - Standardize flag to `groups_backend` in migration flag updates and
comments.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
cdf92fd. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: ds300

apps/dotcom/sync-worker/src/TLUserDurableObject.ts

@@ -111,7 +111,7 @@ export class TLUserDurableObject extends DurableObject<Environment> {
 								exportPadding: true,
 								createdAt: now,
 								updatedAt: now,
-								flags: '',
+								flags: 'groups_backend',
 							})
 							.execute()
 						await tx

apps/dotcom/zero-cache/migrations/023_groups.sql

@@ -404,11 +404,13 @@ BEGIN
 
     -- Create a home group for the user
     INSERT INTO public."group" ("id", "name", "createdAt", "updatedAt", "isDeleted", "inviteSecret")
-    VALUES (target_user_id, target_user_id, v_now, v_now, FALSE, invite_secret);
+    VALUES (target_user_id, target_user_id, v_now, v_now, FALSE, invite_secret)
+    ON CONFLICT DO NOTHING;
 
     -- Make the user a member of the home group
     INSERT INTO public."group_user" ("userId", "groupId", "createdAt", "updatedAt", "role", "index", "userName", "userColor")
-    VALUES (target_user_id, target_user_id, v_now, v_now, 'owner', 'a1', '', '');
+    VALUES (target_user_id, target_user_id, v_now, v_now, 'owner', 'a1', '', '')
+    ON CONFLICT DO NOTHING;
 
     -- For any files owned by the user, change the owningGroupId to the home group and set the ownerId to null
     UPDATE public."file"
@@ -420,7 +422,8 @@ BEGIN
     INSERT INTO public."group_file" ("fileId", "groupId", "createdAt", "updatedAt")
     SELECT fs."fileId", target_user_id, COALESCE(fs."firstVisitAt", f."createdAt", v_now), COALESCE(fs."lastEditAt", fs."firstVisitAt", f."updatedAt", f."createdAt", v_now)
     FROM public."file_state" fs JOIN public."file" f ON fs."fileId" = f."id"
-    WHERE fs."userId" = target_user_id;
+    WHERE fs."userId" = target_user_id
+    ON CONFLICT DO NOTHING;
 
     -- Update indexes for the group_file entries
     UPDATE public."group_file" gf
@@ -433,7 +436,7 @@ BEGIN
     ) AS fs
     WHERE gf."fileId" = fs."fileId" AND gf."groupId" = target_user_id;
 
-    -- Add 'groups' flag to user
+    -- Add 'groups_backend' flag to user
     IF v_current_flags IS NULL OR v_current_flags = '' THEN
         UPDATE public."user" SET flags = 'groups_backend' WHERE id = target_user_id;
     ELSE