Fixes uptime monitoring incorrectly using public URL port instead of
actual backend forward_port for TCP connectivity checks.
Changes:
- Add ProxyHost relationship to UptimeMonitor model
- Update checkHost() to use ProxyHost.ForwardPort
- Add Preload for ProxyHost in getAllMonitors()
- Add diagnostic logging for port resolution
This fixes false "down" status for services like Wizarr that use
non-standard backend ports (5690) while exposing standard HTTPS (443).
Testing:
- Wizarr now shows as "up" (was incorrectly "down")
- All 16 monitors working correctly
- Backend coverage: 85.5%
- No regressions in other uptime checks
Resolves: Wizarr uptime monitoring false negative
