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/docker-compose.yml

@@ -0,0 +1,24 @@
+# Local test forum on http://localhost:8081 (admin / adminadmin). Data in named volumes.
+services:
+  db:
+    image: mariadb:11
+    environment:
+      MARIADB_DATABASE: flarum
+      MARIADB_USER: flarum
+      MARIADB_PASSWORD: flarum
+      MARIADB_RANDOM_ROOT_PASSWORD: "1"
+    volumes:
+      - forum-db:/var/lib/mysql
+  flarum:
+    build: docker
+    depends_on: [db]
+    environment:
+      FORUM_URL: http://localhost:8081
+      ADMIN_PASSWORD: adminadmin
+    ports:
+      - "127.0.0.1:8081:80"
+    volumes:
+      - ./extension:/var/www/flarum/packages/oc-museum
+      - ./import:/import
+volumes:
+  forum-db: