feat(api): add GET /accounts/{address}/positions

[JiahuiWho]

Maps wallet-backend Blend positions into the frontend-shaped response: one payload serves Position Home (header + pool rows) and Position Details (per-asset supply/borrow rows).

• Mapper: fully-exited (all-zero) reserve rows are filtered from display lists (upstream emits them for earnings history); supply rows combine plain-supply and collateral portions while preserving the split; interest_earned_usd converts the token-denominated lifetime interest at today's oracle price (null when price/decimals unavailable, never a fabricated zero).
• Account header: total = sum of pool net values with strict null propagation; net APY = net-value-weighted mean of pool net APYs. Both rules are isolated in accountAggregate() pending upstream confirmation.
• Redis cache per (network, address), TTL from BlendConfig (default 30s); cache reads fail open, cache errors never fail the request.
• Handler follows house patterns: network + strkey validation, 10s timeout, translateServiceError mapping (upstream errors -> 502), empty