T4-020: Daily challenge notification via bot

Track: 4 — Telegram Mini App
Section: Phase B: Health Challenges (40h)
Status: [ ] Todo
Repo: backend-api
Branch: feature/T4-020-daily-challenge-notification
Estimated Hours: 3h

Done When

Bot sends daily progress update to challenge participants: "Day 3/7: You've done 8,234 steps (82% of goal). Keep going!"

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