Implement proxy header injection, hop-by-hop removal, and request forwarding

- Add ProxyError enum with IntoResponse for error handling (400, 404, 502, 504)
- Implement proxy header injection: X-Real-IP, X-Forwarded-For (replaced, not appended), X-Forwarded-Proto
- Implement hop-by-hop header removal for both request and response headers
- Implement request forwarding via shared hyper::Client with HTTP and HTTPS support
- Add ProxyState with http_client and https_client instances shared via axum State
- Add per-site timeout overrides using tokio::time::timeout
- Add HTTPS upstream support with system native TLS root certificates
- No Server or Via headers added to responses
- Host header preserved as-is
- Add unit tests for header injection, hop-by-hop removal, and URI building
- Add integration tests for proxy forwarding, hop-by-hop removal, and 502 on unreachable upstream

src/proxy/mod.rs

@@ -3,3 +3,4 @@ pub mod handler;
 pub mod headers;
 
 pub use crate::config::dynamic_config::normalize_host;
+pub use handler::{create_http_client, create_https_client, proxy_router, ProxyState};