Tweaks and fixes throughout the project (#27)

This PR contains the work done to do a little bit of housekeeping pass across all packages and the Website service.

To provide further details about the work:
* Refreshed the READMEs and source documentation to match the current code;
* Tagged every test case consistently across the Infrastructure, Localization, Persistence, and Website test targets;
* Removed Website middleware tests now covered by Infrastructure's own suite;
* Conformed the `PrepareDB` method to Sendable;
* Relaxes the production Compose DATABASE_TLS default from require to prefer;
* Added Persistence test verifying the prefer posture falls back to plaintext connections.

Reviewed-on: rock-n-code/loud-amsterdam#27
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Packages/Persistence/Tests/Utils/Fakes/PlaintextMySQLServer.swift

@@ -0,0 +1,162 @@
+import NIOCore
+import NIOPosix
+
+/// A fake MySQL server speaking just enough of the wire protocol to complete a plaintext handshake.
+///
+/// Its greeting advertises the `mysql_native_password` plugin but **not** the `CLIENT_SSL` capability, and
+/// it answers the client's handshake response with a bare OK packet — so a client asking for TLS can only
+/// end up connected in plaintext. This is what the `prefer` fallback test connects to, proving the driver
+/// downgrades to plaintext rather than refusing the connection.
+final class PlaintextMySQLServer {
+
+    // MARK: Properties
+
+    /// The port the server listens on, assigned by the system at bind time.
+    let port: Int
+
+    /// The listening channel the server accepts connections through.
+    private let channel: Channel
+
+    // MARK: Initializers
+
+    private init(
+        channel: Channel,
+        port: Int
+    ) {
+        self.channel = channel
+        self.port = port
+    }
+
+    // MARK: Functions
+
+    /// Starts a server on the loopback interface, on a system-assigned port.
+    /// - Returns: the running server, ready to be connected to at ``port``.
+    static func start() async throws -> PlaintextMySQLServer {
+        let channel = try await ServerBootstrap(group: MultiThreadedEventLoopGroup.singleton)
+            .childChannelInitializer { channel in
+                channel.eventLoop.makeCompletedFuture {
+                    try channel.pipeline.syncOperations.addHandler(Handler())
+                }
+            }
+            .bind(host: "127.0.0.1", port: 0)
+            .get()
+
+        guard let port = channel.localAddress?.port else {
+            throw ChannelError.unknownLocalAddress
+        }
+
+        return .init(
+            channel: channel,
+            port: port
+        )
+    }
+
+    /// Stops the server, closing its listening channel.
+    func stop() async throws {
+        try await channel.close().get()
+    }
+
+}
+
+// MARK: - Handlers
+
+private extension PlaintextMySQLServer {
+
+    /// Greets a freshly accepted connection, accepts whatever authentication response arrives,
+    /// and closes on anything after that (e.g. a `COM_QUIT`).
+    final class Handler: ChannelInboundHandler {
+
+        // MARK: Type aliases
+
+        typealias InboundIn = ByteBuffer
+        typealias OutboundOut = ByteBuffer
+
+        // MARK: Properties
+
+        /// Whether the client's handshake response has already been answered with an OK packet.
+        private var didAuthenticate = false
+
+        // MARK: Functions
+
+        func channelActive(context: ChannelHandlerContext) {
+            context.writeAndFlush(
+                wrapOutboundOut(Self.greeting(allocator: context.channel.allocator)),
+                promise: nil
+            )
+        }
+
+        func channelRead(
+            context: ChannelHandlerContext,
+            data: NIOAny
+        ) {
+            guard didAuthenticate else {
+                didAuthenticate = true
+
+                context.writeAndFlush(
+                    wrapOutboundOut(Self.ok(allocator: context.channel.allocator)),
+                    promise: nil
+                )
+
+                return
+            }
+
+            context.close(promise: nil)
+        }
+
+        // MARK: Helpers
+
+        /// The `HandshakeV10` greeting, framed and ready to send as the connection's first packet.
+        ///
+        /// The advertised capabilities are `CLIENT_LONG_PASSWORD`, `CLIENT_PROTOCOL_41`,
+        /// `CLIENT_SECURE_CONNECTION`, and `CLIENT_PLUGIN_AUTH` — deliberately **not** `CLIENT_SSL`,
+        /// so the client cannot upgrade the connection to TLS.
+        private static func greeting(allocator: ByteBufferAllocator) -> ByteBuffer {
+            var payload = allocator.buffer(capacity: 80)
+
+            payload.writeInteger(10, endianness: .little, as: UInt8.self)     // protocol version
+            payload.writeNullTerminatedString("8.0.0")                       // server version
+            payload.writeInteger(1, endianness: .little, as: UInt32.self)    // connection id
+            payload.writeBytes([1, 2, 3, 4, 5, 6, 7, 8])                     // auth plugin data, part 1
+            payload.writeInteger(0, endianness: .little, as: UInt8.self)     // filler
+            payload.writeInteger(0x8201, endianness: .little, as: UInt16.self) // capabilities, lower: LONG_PASSWORD | PROTOCOL_41 | SECURE_CONNECTION
+            payload.writeInteger(0x21, endianness: .little, as: UInt8.self)  // character set (utf8)
+            payload.writeInteger(0x0002, endianness: .little, as: UInt16.self) // status flags (autocommit)
+            payload.writeInteger(0x0008, endianness: .little, as: UInt16.self) // capabilities, upper: PLUGIN_AUTH
+            payload.writeInteger(21, endianness: .little, as: UInt8.self)    // auth plugin data length
+            payload.writeBytes([UInt8](repeating: 0, count: 10))             // reserved
+            payload.writeBytes([9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 0]) // auth plugin data, part 2
+            payload.writeNullTerminatedString("mysql_native_password")       // auth plugin name
+
+            return framed(payload, sequence: 0, allocator: allocator)
+        }
+
+        /// A bare OK packet, framed as the reply to the client's handshake response.
+        private static func ok(allocator: ByteBufferAllocator) -> ByteBuffer {
+            var payload = allocator.buffer(capacity: 8)
+
+            payload.writeBytes([0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00])  // OK, no rows, autocommit, no warnings
+
+            return framed(payload, sequence: 2, allocator: allocator)
+        }
+
+        /// Wraps a payload in the MySQL packet frame: a 3-byte little-endian length and a sequence byte.
+        private static func framed(
+            _ payload: ByteBuffer,
+            sequence: UInt8,
+            allocator: ByteBufferAllocator
+        ) -> ByteBuffer {
+            var packet = allocator.buffer(capacity: payload.readableBytes + 4)
+            var payload = payload
+
+            packet.writeInteger(UInt8(payload.readableBytes & 0xff))
+            packet.writeInteger(UInt8((payload.readableBytes >> 8) & 0xff))
+            packet.writeInteger(UInt8((payload.readableBytes >> 16) & 0xff))
+            packet.writeInteger(sequence)
+            packet.writeBuffer(&payload)
+
+            return packet
+        }
+
+    }
+
+}