fix: resolve 30 test failures across backend and frontend
Backend fixes:
- Add DNS provider registry initialization via blank imports
- Fix credential field name mismatches (hetzner, digitalocean, dnsimple)
- Add comprehensive input validation to security handler
- Resolve certificate deletion database lock with txlock parameter
Frontend fixes:
- LiveLogViewer test timeout already resolved in codebase
Security:
- Zero HIGH/CRITICAL findings in all scans (CodeQL, Trivy, govulncheck)
Test results: All 30 originally failing tests now passing
Coverage: Backend 82.2%, Frontend 84.69% (needs 0.31% increase)
Closes #461
354d15ec5c