Trailing slash middleware for the Infrastructure package (#53)

Services/Website/Sources/App/Extensions/App+Build.swift

@@ -136,7 +136,8 @@ private func logger(
 /// Builds the application's router.
 ///
 /// Registers the request-logging middleware, the security-headers middleware that stamps the given `securityHeaders` onto every response, the
-/// HTTPS-redirect middleware that bounces requests forwarded over plain HTTP to the canonical origin, the vary middleware that marks every response as varying on `Accept-Encoding`, the response-compression middleware that compresses responses
+/// HTTPS-redirect middleware that bounces requests forwarded over plain HTTP to the canonical origin, the trailing-slash redirect middleware that
+/// collapses each path onto its canonical form, the vary middleware that marks every response as varying on `Accept-Encoding`, the response-compression middleware that compresses responses
 /// larger than `minimumResponseSizeToCompress` when the client advertises support, the localization middleware that negotiates the request's
 /// language from its `Accept-Language` header, the not-found middleware that serves the error page, and the static file middleware that serves the
 /// contents of `staticFilesPath` (tagging responses with the given `cacheControl` directives), then adds the `RootController` routes that
@@ -144,7 +145,8 @@ private func logger(
 ///
 /// The security-headers middleware sits just inside request logging so it covers every response that reaches a client — the landing page, the compressed
 /// responses, the rendered not-found page, and the served static files. The HTTPS redirect sits directly beneath it, so a redirect carries the security
-/// headers but skips the negotiation, compression, and file lookup it would otherwise pay for.
+/// headers but skips the negotiation, compression, and file lookup it would otherwise pay for. The trailing-slash redirect follows it, ahead of the
+/// routes and `FileMiddleware` that would otherwise answer both spellings of every path.
 /// - Parameters:
 ///   - staticFilesPath: the folder, relative to the working directory, the static files are served from.
 ///   - assetVersion: the version token the pages append to their asset URLs, or `nil` to leave them unversioned.
@@ -184,6 +186,7 @@ private func router(
         HTTPSRedirectMiddleware(
             configuration: httpsRedirect
         )
+        TrailingSlashRedirectMiddleware()
         VaryMiddleware()
         ResponseCompressionMiddleware(
             minimumResponseSizeToCompress: compressionMinResponseSize