(release) Add Django changelog page

Expose the repository CHANGELOG.md through a staff-only Django view and
link it from the main navigation.

Render a small safe subset of Markdown without adding a runtime dependency,
copy the changelog into the Docker image, and cover the page with view tests.

src/pobsync_server/urls.py

@@ -8,6 +8,7 @@ from pobsync_backend import api, views
 
 urlpatterns = [
     path("", views.dashboard, name="dashboard"),
+    path("changelog/", views.changelog, name="changelog"),
     path("self-check/", views.self_check, name="self_check"),
     path("logs/", views.logs, name="logs"),
     path("config/global/", views.edit_global_config, name="edit_global_config"),