[2.x] fix(realtime): order users by joined_at in realtime:info

[imorland]

Summary

php flarum realtime:info failed with Unknown column 'created_at' in 'order clause' because the command used Eloquent's oldest() without arguments — which defaults to a created_at column that Flarum's users table doesn't have. Flarum uses joined_at instead.

Fixes #4617

Changes

• extensions/realtime/src/Websocket/Console/InfoCommand.php — pass 'joined_at' to oldest() explicitly.

This is the only place in the framework that hits this pattern (verified via grep).

Why not override CREATED_AT on User or AbstractModel

• AbstractModel is the base for every Flarum model — most use created_at, only User uses joined_at. There's no single column name that fits all.
• Setting const CREATED_AT = 'joined_at' on User would help future callers but makes the model look like it has Eloquent-style timestamps when it doesn't ($timestamps = false). Risk of confusing third-party code that introspects the model.
• The targeted fix at the call site is clearer about what the code wants (oldest user by join date) and matches Flarum's existing convention everywhere else.

Test plan

• Manually verified php flarum realtime:info runs to completion in the dev container.
• No automated test added — the command depends on a live Pusher connection, so writing one would require heavy mocking for a one-line fix.