feat(apis): Redis-backed cache for market and position data

[IbrahimIjai]

Context

The current market/position cache is in-process (Arc<RwLock>). This means:

• Cache is lost on every server restart (cold boot hits Stellar RPC with N markets x M calls)
• Running multiple API replicas behind a load balancer means each has a stale independent cache

Replacing with Redis gives persistence across restarts and a shared cache across replicas.

Implementation

Add a Redis client (using deadpool-redis) as an AppState dependency.

Cache keys and TTLs:

• markets_list -> 30s
• market:{address} -> 15s
• positions:{account} -> 10s
• orders:{account} -> 10s
• stats -> 60s

Fallback: if Redis is unreachable on startup, fall back to in-memory cache and log a warning (degraded mode, not a hard failure).

Env vars: REDIS_URL (default redis://localhost:6379), REDIS_CACHE_ENABLED=true|false.

Acceptance criteria

• All cached endpoints read from and write to Redis
• TTLs enforced by Redis EXPIRE, not application-level timers
• Graceful degradation to in-memory cache when Redis is unavailable
• docker-compose.yml already includes Redis service — wire it into apis startup
• Integration test: populate Redis cache, restart server, verify cache hit
• REDIS_CACHE_ENABLED=false disables Redis entirely (useful for local dev without Docker)