#10 Issue 10: [Performance] Redis Cache for Meter State

[elizabetheonoja-art]

Don't query Postgres for every single MQTT heartbeat. Cache device state in Redis for speed.

Acceptance Criteria:

[ ] Key: meter:{id}:state.

[ ] Value: { isActive: true, balance: 100 }.

[ ] TTL: 60 seconds.

Priority: Medium
Labels: performance, caching