Move Bloom participant flows out of Blade

[DVidal1205]

Goal

Make BloomKnights own participant authentication, application, dashboard, and profile flows while Blade remains member- and organizer-facing.

Implementation

• Add a reusable headless @forge/hackathon toolkit and participant-scoped tRPC router.
• Add app-configurable Better Auth support with Bloom-local OAuth routes.
• Move Bloom application, dashboard, and profile experiences to bloom.knighthacks.org.
• Add hackathon portal URL and confirmation capacity configuration.
• Replace Blade participant pages with compatibility redirects and retain Blade admin workflows.
• Port the Bloom-specific visual work from blade/bloomknights-dashboard-theme with attribution.

Acceptance criteria

• Bloom owns /apply, /dashboard, and /dashboard/profile with local Discord auth.
• Blade /dashboard is member-only and no longer displays the live-hackathon chooser.
• Participant mutations are scoped to the authenticated user and explicit hackathon.
• Existing Blade admin hackathon workflows remain functional.
• Legacy participant URLs redirect to the configured external portal.