Reliable delivery: per-device queues and retries

[fabianabarca]

Description

Ensure message persistence and retry strategy.

Acceptance Criteria

• Queue per device with backoff
• At-least-once semantics
• Metrics for delivery

[fabianabarca]

Tracked in #1

[gonguma]

Starts: 2025-10-22
Ends: 2025-10-26
Depends on: #3. To ensure reliable delivery and handle retries, the persistent connections must be working properly first.