Move email sending to background job pipeline with retry/backoff

[ganeshwhere]

Objective

Make email delivery more reliable and observable by moving to background processing.

Scope

• Queue invite and task-assignment emails
• Retry with exponential backoff
• Dead-letter handling and delivery audit signals

Acceptance Criteria

• Server endpoints enqueue jobs and return immediately
• Transient failures are retried automatically
• Permanent failures are captured and visible in logs/metrics
• Unit/integration tests verify queue and retry behavior

Non-Goals

• Multi-provider email failover