IMG-014: No Retry Logic for Downloads

[murdore]

Summary

Network downloads fail permanently on first error without retry.

Root Cause

Lines 524-573 single attempt for URL downloads.

Fix

Implement retry wrapper with exponential backoff (3 attempts, 1s/2s/4s delays).

Acceptance Criteria

• [ ] Implement retry wrapper with exponential backoff
• [ ] Maximum 3 attempts
• [ ] Only retry on network errors, not 404/403
• [ ] Add tests for retry success and eventual failure