T4-009: Challenge data model — create challenges table

Track: 4 — Telegram Mini App
Section: Phase B: Health Challenges (40h)
Status: [ ] Todo
Repo: backend-api
Branch: feature/T4-009-challenges-table
Estimated Hours: 3h

Done When

Table: id, title, description, type (steps/sleep/workout/custom), target_value, start_date, end_date, created_by, max_participants, reward_pool, status. Migration runs

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