Add image auto-register retry

[ckcoding]

Summary

• add bounded auto-register retry when image account selection has no usable quota
• serialize concurrent auto-register attempts and wait for the in-flight registration result
• log auto-register supplement/success/failure/skip/exhaustion events
• document the 80/666 server deployments and current register provider configuration

Tests

• uv run python -m unittest test.test_account_image_capabilities
• uv run python -m compileall services/account_service.py test/test_account_image_capabilities.py