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/Middlewares/NotFoundMiddleware.swift

@@ -1,6 +1,4 @@
-import Elementary
 import Hummingbird
-import HummingbirdElementary
 
 /// Serves a custom error page for requests that match neither a route nor a static file.
 ///
@@ -9,10 +7,20 @@ import HummingbirdElementary
 /// ``ErrorPage`` and a `404 Not Found` status.
 public struct NotFoundMiddleware<Context: RequestContext> {
 
+    // MARK: Properties
+
+    /// The error page, rendered once at initialization and reused for every not-found response.
+    private let cache: CachedHTMLResponse
+
     // MARK: Initializers
 
     /// Creates a not-found middleware.
-    public init() {}
+    public init() {
+        self.cache = .init(
+            status: .notFound,
+            ErrorPage()
+        )
+    }
 
 }
 
@@ -39,7 +47,8 @@ extension NotFoundMiddleware: RouterMiddleware {
     ) async throws -> Response {
         do {
             return try await next(request, context)
-        } catch let error {
+        }
+        catch let error {
             guard
                 let responseError = error as? any HTTPResponseError,
                 responseError.status == .notFound
@@ -47,15 +56,7 @@ extension NotFoundMiddleware: RouterMiddleware {
                 throw error
             }
 
-            return HTMLResponse(
-                status: .notFound
-            ) {
-                ErrorPage()
-            }
-            .response(
-                from: request,
-                context: context
-            )
+            return cache.response()
         }
     }