feat(user): add welcome-seen-at timestamp for welcome page tracking

Null means the welcome page was not seen yet. Existing users are backfilled
at migration time so only newly registered users see the page.

readme.md

@@ -78,6 +78,9 @@ To Be Played is split into three repositories:
   drafts). The dashboard shows the entries published after `user_settings.news_seen_at` once in a dialog; "Got it"
   sets the stamp to now (`PUT /api/news/seen`). A new user starts at the registration time. `/whats-new` lists all
   entries. Editing an entry does not make it unseen again. Create, update and delete are rows in `admin_audit`.
+- **Welcome page**: `user_settings.welcome_seen_at` is null until the user sees the welcome page; "Let's go" and "Skip"
+  set it (`PUT /api/users/settings/welcome-seen`). V24 sets it for every existing user, so only users registered after
+  the deploy get the page.
 - **Mails** from the platform mailbox: "a new registration waits" to the admin (`APP_MAIL_ADMIN_TO`, with the link
   to the admin page), and to the user "we got your registration", then "your account is active" or "your
   registration was denied", later "your account is blocked" and "your account will be deleted from <date>". The registration sends the browser language (`de` for a German browser, else `en`), which also becomes
@@ -123,6 +126,7 @@ To Be Played is split into three repositories:
 | `PUT` | `/api/users/settings/appearance` | access token |
 | `PUT` | `/api/users/settings/profile-style` (`{accent, corners, bannerAppid, backgroundAppid}`; `204`) | access token |
 | `PUT` | `/api/users/settings/dashboard-tiles` (`{tiles: [{id, size}]}` in order, at most 20; `{tiles: null}` resets; `204`). Auth checks only the id shape `[A-Z_]{1,30}`: Core and the Frontend skip ids they do not know, and Core never shares the overview or the activity | access token |
+| `PUT` | `/api/users/settings/welcome-seen` (`204`; stamps `user_settings.welcome_seen_at`, sent as `settings.welcomeSeenAt` in `/me`, null until then) | access token |
 | `PUT` | `/api/users/settings/favorite-platform` (`{platform}`, free text up to 32, e.g. `PS5`; `null` clears it; `204`) | access token |
 | `PUT` | `/api/users/settings/profile-tiles` (`{tiles: [{id, size}]}`, e.g. `SUMMARY`, `STATUS`, `PLAYING`, `FINISHED`, `STATS_<GROUP>`; `204`) | access token |
 | `GET` | `/api/public/profiles/{displayName}` | public |