refactor(config): centralize env loading and enforce typed access

- Centralize environment variable loading in config.MustLoad
- Parse and validate all env vars once at initialization
- Make config fields private and read-only
- Remove public Getenv usage in favor of typed accessors
- Improve validation and initialization order
- Normalize enum naming to be idiomatic and avoid constant collisions

internal/transport/http.go

@@ -12,9 +12,9 @@ type httpServer struct {
 	port    string
 }
 
-func NewHTTPServer(port string, sessionRegistry registry.Registry, redirectTLS bool) Transport {
+func NewHTTPServer(domain, port string, sessionRegistry registry.Registry, redirectTLS bool) Transport {
 	return &httpServer{
-		handler: newHTTPHandler(sessionRegistry, redirectTLS),
+		handler: newHTTPHandler(domain, sessionRegistry, redirectTLS),
 		port:    port,
 	}
 }