T4-002: Bot webhook server — receive and handle Telegram updates

Track: 4 — Telegram Mini App
Section: Phase A: Bot & Mini App Foundation (24h)
Status: [ ] Todo
Repo: backend-api
Branch: feature/T4-002-bot-webhook-server
Estimated Hours: 4h

Done When

Webhook endpoint receives messages. Bot responds to /start, /help, /challenges. Validates Telegram signature

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