fix: retry coordinator startup connection

[Gerrri]

Summary

• retry coordinator connections in the background when startup begins in degraded mode
• restore target subscriptions and broadcast a fresh targets list after recovery
• cover the reconnect lifecycle with backend tests and document the behavior in the staging README

Notes

• Fixes Add automatic coordinator reconnect after startup failure #23

Verification

• DEBUG=false python -m pytest tests/test_main.py tests/test_health.py (run from backend/)
• docker compose --profile staging up -d --build
• curl -sf http://localhost:8000/api/health