Implement complete workflow for assigning security header profiles
to proxy hosts via dropdown selector in ProxyHostForm.
Backend Changes:
- Add security_header_profile_id handling to proxy host update endpoint
- Add SecurityHeaderProfile preloading in service layer
- Add 5 comprehensive tests for profile CRUD operations
Frontend Changes:
- Add Security Headers section to ProxyHostForm with dropdown
- Group profiles: System Profiles (presets) vs Custom Profiles
- Remove confusing "Apply" button from SecurityHeaders page
- Rename section to "System Profiles (Read-Only)" for clarity
- Show security score inline when profile selected
UX Improvements:
- Clear workflow: Select profile → Assign to host → Caddy applies
- No more confusion about what "Apply" does
- Discoverable security header assignment
- Visual distinction between presets and custom profiles
Tests: Backend 85.6%, Frontend 87.21% coverage
Docs: Updated workflows in docs/features.md
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