Static file caching for the Website service (#6)

This PR contains the work done to add cache-control headers to static file responses by configuring the `FileMiddleware` middleware to tag served static files with *Cache-Control* directives, tuned per media type.

To provide further details about the work done:

* Added a cache control that sets per-type policies: text assets (CSS/JS) get public, max-age, must-revalidate; images get public, max-age; everything else gets a default public, max-age.
* Files stay validated via `ETag/Last-Modified` header.
* Made the max-age values configurable, with defaults, via new cache constants.

Reviewed-on: rock-n-code/loud-amsterdam#6
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Tests/App/AppTests.swift

@@ -13,6 +13,12 @@ import Testing
 struct AppTests {
 
     // MARK: Constants
+    
+    private let textExtensions: [StaticFile.Extension] = [
+        .css,
+        .js,
+        .txt
+    ]
 
     // Absolute path to the package's "Resources/Static" folder, derived from this
     // file's location so the static files resolve regardless of the working directory.
@@ -35,7 +41,7 @@ struct AppTests {
                 let body = String(buffer: response.body)
 
                 #expect(response.status == .ok)
-                #expect(response.headers[.contentType]?.hasPrefix("text/html") == true)
+                #expect(response.headers[.contentType] == "text/html; charset=utf-8")
                 #expect(body.contains("Hello world!"))
             }
         }
@@ -52,6 +58,15 @@ struct AppTests {
             ) { response in
                 #expect(response.status == .ok)
                 #expect(response.headers[.contentType] == file.contentType)
+                
+                let cacheControl = try #require(response.headers[.cacheControl])
+
+                #expect(cacheControl.contains("public") == true)
+                #expect(cacheControl.contains("max-age=") == true)
+                
+                if textExtensions.contains(file.fileExtension) {
+                    #expect(cacheControl.contains("must-revalidate") == true)
+                }
             }
         }
     }
@@ -66,7 +81,7 @@ struct AppTests {
                 let body = String(buffer: response.body)
 
                 #expect(response.status == .notFound)
-                #expect(response.headers[.contentType]?.hasPrefix("text/html") == true)
+                #expect(response.headers[.contentType] == "text/html; charset=utf-8")
                 #expect(body.contains("Page Not Found"))
             }
         }