Cache GET /requests/:id with Redis

[iexwr]

Context

A request row is read frequently (dashboard polls, webhook consumers) but changes rarely. Hitting Postgres every time is wasteful.

Acceptance criteria

• Cache the serialized request under req:<contract>:<id> with a configurable TTL (env CACHE_TTL, default 15s).
• Invalidate the cache on write from the indexer (paid/cancelled).
• Falls back to DB on cache miss / Redis error; never hard-fail.

Technical notes

• Use the existing ioredis client.
• Keep TTL short; state changes must reflect quickly.

Testing

• Test that a cache hit returns DB data and a subsequent invalidation forces a refresh.