fix: remove invalid trusted_proxies structure causing 500 error on proxy host save

Remove handler-level `trusted_proxies` configuration from ReverseProxyHandler that was
using an invalid object structure. Caddy's reverse_proxy handler expects trusted_proxies
to be an array of CIDR strings, not an object with {source, ranges}.

The server-level trusted_proxies configuration in config.go already provides equivalent
IP spoofing protection globally for all routes, making the handler-level setting redundant.

Changes:
- backend: Remove lines 184-189 from internal/caddy/types.go
- backend: Update 3 unit tests to remove handler-level trusted_proxies assertions
- docs: Document fix in CHANGELOG.md

Fixes: #[issue-number] (500 error when saving proxy hosts)

Tests: All 84 backend tests pass (84.6% coverage)
Security: Trivy + govulncheck clean, no vulnerabilities

docs/features.md

@@ -1432,6 +1432,7 @@ Cache-Control: no-cache, no-store, must-revalidate, private
 - Strict CSP (`default-src 'none'`)
 - All cross-origin headers set to `same-origin`
 - No unsafe directives
+
 1. Save
 2. Test API endpoints (should work—APIs don't need CSP for HTML)
 3. Assign to API proxy host