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/http/header/header.go

@@ -17,9 +17,9 @@ type RequestHeader interface {
 	Set(key string, value string)
 	Remove(key string)
 	Finalize() []byte
-	GetMethod() string
-	GetPath() string
-	GetVersion() string
+	Method() string
+	Path() string
+	Version() string
 }
 type requestHeader struct {
 	method    string