Add email notification deduplication

[meshackyaro]

Description

Users receive multiple duplicate notification emails for same event. Event handler registered multiple times.

Component

Backend

Difficulty

🟡 Medium

Tasks

• Implement message ID-based deduplication
• Use Redis for deduplication cache
• Add idempotency keys
• Test duplicate prevention
• Log duplicate attempts

Acceptance Criteria

• One email per event
• Duplicates detected and prevented
• Deduplication logged

Estimated Time

1-2 days