refactor(api): move auth middleware to project root

PluralKit.API/AuthorizationTokenHandlerMiddleware.cs

@@ -0,0 +1,32 @@
+using Dapper;
+
+using PluralKit.Core;
+
+namespace PluralKit.API;
+
+public class AuthorizationTokenHandlerMiddleware
+{
+    private readonly RequestDelegate _next;
+
+    public AuthorizationTokenHandlerMiddleware(RequestDelegate next)
+    {
+        _next = next;
+    }
+
+    public async Task Invoke(HttpContext ctx, IDatabase db)
+    {
+        ctx.Request.Headers.TryGetValue("authorization", out var authHeaders);
+        if (authHeaders.Count > 0)
+        {
+            var systemId = await db.Execute(conn => conn.QuerySingleOrDefaultAsync<SystemId?>(
+                "select id from systems where token = @token",
+                new { token = authHeaders[0] }
+            ));
+
+            if (systemId != null)
+                ctx.Items.Add("SystemId", systemId);
+        }
+
+        await _next.Invoke(ctx);
+    }
+}