feat: add some fediverse authentication code

* create applications on instances
* generate authorize URLs
* exchange oauth code for token and user info (untested)
* recreate mastodon app on authentication failure

Foxnouns.Backend/Controllers/Authentication/FediverseAuthController.cs

@@ -0,0 +1,25 @@
+using Foxnouns.Backend.Services;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Foxnouns.Backend.Controllers.Authentication;
+
+[Route("/api/internal/auth/fediverse")]
+public class FediverseAuthController(FediverseAuthService fediverseAuthService) : ApiControllerBase
+{
+    [HttpGet]
+    [ProducesResponseType<FediverseUrlResponse>(statusCode: StatusCodes.Status200OK)]
+    public async Task<IActionResult> GetFediverseUrlAsync([FromQuery] string instance)
+    {
+        var url = await fediverseAuthService.GenerateAuthUrlAsync(instance);
+        return Ok(new FediverseUrlResponse(url));
+    }
+
+    public async Task<IActionResult> FediverseCallbackAsync([FromBody] CallbackRequest req)
+    {
+        throw new NotImplementedException();
+    }
+
+    public record CallbackRequest(string Instance, string Code);
+
+    private record FediverseUrlResponse(string Url);
+}