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/Public/Controllers/RootController.swift

@@ -1,5 +1,4 @@
 import Hummingbird
-import HummingbirdElementary
 
 /// Serves the website's root routes.
 ///
@@ -12,14 +11,21 @@ import HummingbirdElementary
 ///
 /// - Note: `Context` is the request context the routes are resolved against, and must match the
 ///   context of the router the routes are added to.
-public struct RootController<Context: RequestContext> : Sendable{
+public struct RootController<Context: RequestContext>: Sendable {
+
+    // MARK: Properties
+
+    /// The landing page, rendered once at initialization and reused for every request.
+    private let cache: CachedHTMLResponse
 
     // MARK: Initializers
 
     /// Creates a root controller.
-    public init() {}
+    public init() {
+        self.cache = .init(IndexPage())
+    }
 
-    // MARK: Properties
+    // MARK: Computed
 
     /// The routes served by the controller.
     ///
@@ -40,29 +46,27 @@ public struct RootController<Context: RequestContext> : Sendable{
 // MARK: - Helpers
 
 private extension RootController {
-    
+
     // MARK: Methods
 
     /// Handles a request for the landing page.
     /// - Parameters:
     ///   - request: the incoming request.
     ///   - context: the context the request is resolved against.
-    /// - Returns: an HTML response that renders the ``IndexPage``.
+    /// - Returns: the cached ``IndexPage`` response.
     @Sendable
     func index(
         request: Request,
         context: some RequestContext
-    ) -> HTMLResponse {
-        .init {
-            IndexPage()
-        }
+    ) -> Response {
+        cache.response()
     }
-    
+
 }
 
 // MARK: - Constants
 
-extension RouterPath {
+private extension RouterPath {
     /// A namespace for the ``RootController`` route paths.
     enum Root {
         /// The path of the landing page.