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/Database/Migrations/20241006125003_AddFediverseAccessTokens.cs

@@ -0,0 +1,40 @@
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using NodaTime;
+
+#nullable disable
+
+namespace Foxnouns.Backend.Database.Migrations
+{
+    /// <inheritdoc />
+    [DbContext(typeof(DatabaseContext))]
+    [Migration("20241006125003_AddFediverseAccessTokens")]
+    public partial class AddFediverseAccessTokens : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "access_token",
+                table: "fediverse_applications",
+                type: "text",
+                nullable: true
+            );
+
+            migrationBuilder.AddColumn<Instant>(
+                name: "token_valid_until",
+                table: "fediverse_applications",
+                type: "timestamp with time zone",
+                nullable: true
+            );
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(name: "access_token", table: "fediverse_applications");
+
+            migrationBuilder.DropColumn(name: "token_valid_until", table: "fediverse_applications");
+        }
+    }
+}