Indexer: GET /v1/arena/wallet/{{address}}/stats for profile modal

Context

Wallet stats API for participant profile modal. Parent epic: #238 (closed).

Relevant files

• indexer/src/api.rs
• SQL aggregations over buy, podium payout, XP, CRED, WarBow epochs

Acceptance criteria

API returns:

• epochs_participated (Last Buy epochs with ≥1 buy)
• prizes_won[]: { podium, epoch, rank, amount_doub }
• total_won_doub, total_spent_doub, buy_count
• xp, level
• highest_scores[]: { podium, epoch, score, rank } for Streak, TimeBooster, WarBow, LastBuy
• Bonus stats: first buy timestamp, longest defended streak, WarBow steals/guards, referral CRED earned, CRED claimed, average buy size, max single buy, podium win rate, rank distribution

Verification checklist

• Integration test with fixture buys across 2 Last Buy epochs
• p95 query latency acceptable on Anvil-rich dataset