refactor(backend): replace coravel with hangfire for background jobs

for *some reason*, coravel locks a persistent job queue behind a
paywall. this means that if the server ever crashes, all pending jobs
are lost. this is... not good, so we're switching to hangfire for that
instead.

coravel is still used for emails, though.

BREAKING CHANGE: Foxnouns.NET now requires Redis to work. the EFCore
storage for hangfire doesn't work well enough, unfortunately.

Foxnouns.Backend/Database/Migrations/20250304155708_RemoveTemporaryKeys.cs

@@ -0,0 +1,55 @@
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using NodaTime;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+#nullable disable
+
+namespace Foxnouns.Backend.Database.Migrations
+{
+    /// <inheritdoc />
+    [DbContext(typeof(DatabaseContext))]
+    [Migration("20250304155708_RemoveTemporaryKeys")]
+    public partial class RemoveTemporaryKeys : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(name: "temporary_keys");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "temporary_keys",
+                columns: table => new
+                {
+                    id = table
+                        .Column<long>(type: "bigint", nullable: false)
+                        .Annotation(
+                            "Npgsql:ValueGenerationStrategy",
+                            NpgsqlValueGenerationStrategy.IdentityByDefaultColumn
+                        ),
+                    expires = table.Column<Instant>(
+                        type: "timestamp with time zone",
+                        nullable: false
+                    ),
+                    key = table.Column<string>(type: "text", nullable: false),
+                    value = table.Column<string>(type: "text", nullable: false),
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("pk_temporary_keys", x => x.id);
+                }
+            );
+
+            migrationBuilder.CreateIndex(
+                name: "ix_temporary_keys_key",
+                table: "temporary_keys",
+                column: "key",
+                unique: true
+            );
+        }
+    }
+}