Optimizations for the Website service (#9)

This PR contains the work done to provide optimizations to the current service, such as a health-check endpoint, pre-renders static HTML pages, and hardens the error page's CSP.

To provide further details about the work:

* Added the `HealthController` controller serving GET `/health` with a static JSON payload.
* Added the `CachedHTMLResponse` response, which renders a static HTMLDocument to bytes once and reuses them per request (no Content-Length, so responses stay compressible).
* Integrated the response into the `RootController` and the  `NotFoundMiddleware` middleware to avoid re-rendering on hot paths.
* Added a `RouterMethods.addRoutes(_:)` extension and switched the router in App+build to use it.
* Moved the inline style from the `ErrorPage` page into a dedicated style file so the CSP needs no inline-style escape hatch.
* Fixed the `IndexPage` page path inconsistencies.
* Written the `README` file.

Reviewed-on: rock-n-code/loud-amsterdam#9
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/Library/Internal/Enumerations/StaticFile.swift

@@ -5,6 +5,8 @@
 enum StaticFile: CaseIterable, Sendable {
     /// The `js/app.js` script.
     case appJS
+    /// The `css/error.css` stylesheet for the not-found page.
+    case errorCSS
     /// The `favicon.ico` icon.
     case faviconICO
     /// The `icon.png` icon.
@@ -63,7 +65,8 @@ extension StaticFile {
     /// The file's extension.
     var fileExtension: Extension {
         switch self {
-        case .styleCSS: .css
+        case .errorCSS,
+             .styleCSS: .css
         case .appJS: .js
         case .faviconICO: .ico
         case .iconPNG: .png
@@ -77,6 +80,7 @@ extension StaticFile {
     var fileName: String {
         switch self {
         case .appJS: "app"
+        case .errorCSS: "error"
         case .faviconICO: "favicon"
         case .iconPNG,
              .iconSVG: "icon"
@@ -102,7 +106,7 @@ extension StaticFile {
     /// 
     /// - Parameter basePath: the directory the static files are served from.
     /// - Returns: the path to the file, relative to the `basePath` path.
-    public func path(
+    func path(
         relativeTo basePath: String
     ) -> String {
         guard !basePath.isEmpty else {
@@ -124,7 +128,8 @@ private extension StaticFile {
     var subdirectory: String? {
         switch self {
         case .appJS: "js"
-        case .styleCSS: "css"
+        case .errorCSS,
+             .styleCSS: "css"
         default: nil
         }
     }