fix: add missing field handlers in proxy host Update endpoint

Add handlers for enable_standard_headers, forward_auth_enabled, and waf_disabled fields
in the proxy host Update function. These fields were defined in the model but were not
being processed during updates, causing:

- 500 errors when saving proxy host configurations
- Auth pass-through failures for apps like Seerr/Overseerr due to missing X-Forwarded-* headers

Changes:
- backend: Add field handlers for 3 missing fields in proxy_host_handler.go
- backend: Add 5 comprehensive unit tests for field handling
- frontend: Update TypeScript ProxyHost interface with missing fields
- docs: Document fixes in CHANGELOG.md

Tests: All 1147 tests pass (backend 85.6%, frontend 87.7% coverage)
Security: No vulnerabilities (Trivy + govulncheck clean)

Fixes #16 (auth pass-through)
Fixes #17 (500 error on save)

frontend/src/api/proxyHosts.ts

@@ -33,6 +33,8 @@ export interface ProxyHost {
   block_exploits: boolean;
   websocket_support: boolean;
   enable_standard_headers?: boolean;
+  forward_auth_enabled?: boolean;
+  waf_disabled?: boolean;
   application: ApplicationPreset;
   locations: Location[];
   advanced_config?: string;