fix(session): Add null check for boardId in createSession

[solracsf]

Summary

Exception
OCA\Deck\Controller\SessionController::create(): Argument #1 ($boardId) must be of type int, null given

Remove createSession() from created() entirely and move the single call site to fetchData(), after loadBoardById has successfully resolved — guaranteeing this.id is a valid integer.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required