Forum integration: Flarum extension, local test stack, embedded comments

- forum/extension: oc:import command (archived comments -> one thread per
  machine) and embed styling
- forum/docker: local Flarum 2 test stack
- site: optional params.forum.url adds a Forum menu entry and shows the
  machine thread in the Comments tab

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDRjtmAiNpo5mK5N3d7QwZ

forum/extension/migrations/2026_10_11_000000_create_oc_threads_table.php

@@ -0,0 +1,10 @@
+<?php
+
+use Flarum\Database\Migration;
+use Illuminate\Database\Schema\Blueprint;
+
+// One discussion per museum machine; oc_id is the machine id of the original site.
+return Migration::createTable('oc_threads', function (Blueprint $table) {
+    $table->unsignedInteger('oc_id')->primary();
+    $table->unsignedInteger('discussion_id')->unique();
+});