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/Pages/ErrorPage.swift

@@ -11,14 +11,17 @@ struct ErrorPage: HTMLDocument, Sendable {
         p { "Sorry, but the page you were trying to view does not exist." }
     }
 
-    /// The metadata and inline styles placed in the document head.
+    /// The metadata and stylesheet link placed in the document head.
     var head: some HTML {
         meta(.charset(.utf8))
         meta(
             .name(.viewport),
             .content("width=device-width, initial-scale=1")
         )
-        style { Self.styles }
+        link(
+            .rel(.stylesheet),
+            .href("/css/error.css")
+        )
     }
 
     /// The document language.
@@ -28,52 +31,3 @@ struct ErrorPage: HTMLDocument, Sendable {
     var title: String { "Page Not Found" }
 
 }
-
-// MARK: - Constants
-
-private extension ErrorPage {
-    /// The inline CSS applied to the page.
-    static let styles = """
-        * {
-          line-height: 1.2;
-          margin: 0;
-        }
-
-        html {
-          color: #888;
-          display: table;
-          font-family: sans-serif;
-          height: 100%;
-          text-align: center;
-          width: 100%;
-        }
-
-        body {
-          display: table-cell;
-          vertical-align: middle;
-          margin: 2em auto;
-        }
-
-        h1 {
-          color: #555;
-          font-size: 2em;
-          font-weight: 400;
-        }
-
-        p {
-          margin: 0 auto;
-          width: 280px;
-        }
-
-        @media only screen and (max-width: 280px) {
-          body, p {
-            width: 95%;
-          }
-
-          h1 {
-            font-size: 1.5em;
-            margin: 0 0 0.3em;
-          }
-        }
-        """
-}