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/HealthController.swift

@@ -0,0 +1,83 @@
+import Hummingbird
+import NIOCore
+
+/// Serves the website's health-check route.
+///
+/// The controller exposes its routes as a `RouteCollection` so they can be added to a router
+/// (or a sub-group) by the application that composes it:
+///
+/// ```swift
+/// router.addRoutes(HealthController<AppRequestContext>().routes)
+/// ```
+///
+/// - 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 HealthController<Context: RequestContext>: Sendable {
+    
+    // MARK: Properties
+    
+    /// The JSON payload returned for every health check.
+    private let payload: String
+
+    // MARK: Initializers
+
+    /// Creates a health controller.
+    public init() {
+        self.payload = #"{"status":"ok"}"#
+    }
+
+    // MARK: Computed
+
+    /// The routes served by the controller.
+    ///
+    /// Serves a `GET` request for the health path (`/health`) with a static JSON status payload.
+    public var routes: RouteCollection<Context> {
+        let routes = RouteCollection(context: Context.self)
+
+        routes.get(
+            .Health.check,
+            use: check
+        )
+
+        return routes
+    }
+
+}
+
+// MARK: - Helpers
+
+private extension HealthController {
+
+    // MARK: Methods
+
+    /// Handles a request for the health check.
+    ///
+    /// Returns a constant JSON body built directly per request — the payload is a tiny literal with no
+    /// rendering step, so there is nothing to pre-render or cache.
+    /// - Parameters:
+    ///   - request: the incoming request.
+    ///   - context: the context the request is resolved against.
+    /// - Returns: a `200 OK` response carrying the static JSON status payload.
+    @Sendable
+    func check(
+        request: Request,
+        context: some RequestContext
+    ) -> Response {
+        Response(
+            status: .ok,
+            headers: [.contentType: "application/json"],
+            body: .init(byteBuffer: .init(string: payload))
+        )
+    }
+
+}
+
+// MARK: - Constants
+
+private extension RouterPath {
+    /// A namespace for the ``HealthController`` route paths.
+    enum Health {
+        /// The path of the health-check endpoint.
+        static let check: RouterPath = "/health"
+    }
+}