Add bingo leaderboard API with live Pusher updates

[rxmox]

Summary

Adds the bingo leaderboard backend for the web organizer dashboard, giving real-time participant rankings during events.

New Endpoints

• GET /api/events/:eventId/leaderboard - Returns sorted leaderboard with participant name, profile photo, connections count, lines completed, and completion status
• PUT /api/events/:eventId/leaderboard/score - Lets participants update their bingo score (lines completed, sheet completion)

Leaderboard Display Format (per row)

Column	Description
User	Profile photo + name
Connections	Unique people connected with during bingo (computed from ParticipantConnection records)
Lines Completed	Vertical, horizontal, and diagonal lines
Completed	Boolean flag for when entire sheet is filled (frontend can use for golden/ice-shattering visual)

Design Decisions

• Connections count is computed at query time from existing ParticipantConnection records rather than stored, which avoids sync bugs and keeps data consistent
• A<->B connections (regardless of direction) count as 1 unique partner per participant
• Sort order: completed first, then most lines, then most connections
• Client-trusted scores: mobile pushes linesCompleted and completed, no server-side game state verification needed for MVP

Real-Time Updates

• leaderboard-updated Pusher event fires on score updates (from leaderboard controller) and connection create/delete (from participant connections controller)
• Web dashboard subscribes and re-fetches the full leaderboard on each event

Files Changed

• New: src/controllers/leaderboard_controller.ts
• Modified: src/models/participant_model.ts (added linesCompleted, completed fields)
• Modified: src/routes/event_routes.ts (wired 2 new routes)
• Modified: src/controllers/participant_connections_controller.ts (Pusher triggers on create/delete)
• Docs: Updated API reference, database schema, and real-time events guide