Accept a pending challenge with selected reward.

Changes challenge status from "pending" to "active" and sets:

• selected_reward
• reward_amount (from selected_reward)
• reward_type (derived from selected_reward.tag)
• start_timestamp = now
• end_timestamp = now + time_window_days
• accepted_timestamp = now

Args: request: Contains selected_reward (id, title, tag, amount)

Returns: Updated challenge object with status "active"

Raises: HTTPException: 404 if no pending challenge, 400 if invalid reward, etc.