Convert backend workers to TypeScript

[trillium]

Overview

Convert the backend worker modules (openCheckins, closeCheckins, createRecurringEvents, slackbot, tokenCleanup) to TypeScript to catch type errors at compile time — particularly around date/time handling which has been a source of bugs (#1872).

Action Items

• Add TypeScript and required @types packages to backend
• Create tsconfig.json for backend
• Define types for Event, CheckIn, RecurringEvent models
• Convert worker files from .js to .ts
• Update build/dev scripts to handle TypeScript compilation
• Ensure compatibility with ESM migration (Migrate backend from CommonJS to ES Modules #2133)

Resources/Instructions

• Depends on: Migrate backend from CommonJS to ES Modules #2133 (ESM migration) should be merged first
• Related: Investigate and Fix Incorrect Event Check-In Times in VRMS #1872 (timezone bug in workers)