[Feature] Implement Retry Logic for Notifications #11

Problem

Failed SMS/email deliveries are not retried. Critical notifications can fail silently.

Impact

• Missed emergency notifications
• Coordinators don't get alerted
• No visibility into failures

Solution

Implement exponential backoff:

• Retry up to 3 times
• Delay: 1s, 4s, 9s
• Log retry attempts
• Send alert after final failure

Acceptance Criteria

• Retry logic with exponential backoff
• Max retry limit configurable
• Retry history tracked
• Final failure alerts coordinator