Monorepo: consolidate 7 repos into one

Combines shared, blog, market, cart, events, federation, and account
into a single repository. Eliminates submodule sync, sibling model
copying at build time, and per-app CI orchestration.

Changes:
- Remove per-app .git, .gitmodules, .gitea, submodule shared/ dirs
- Remove stale sibling model copies from each app
- Update all 6 Dockerfiles for monorepo build context (root = .)
- Add build directives to docker-compose.yml
- Add single .gitea/workflows/ci.yml with change detection
- Add .dockerignore for monorepo build context
- Create __init__.py for federation and account (cross-app imports)

events/templates/fragments/container_nav_calendars.html

@@ -0,0 +1,10 @@
+{# Calendar links nav — served as fragment from events app #}
+{% for calendar in calendars %}
+  {% set local_href=events_url('/' + post_slug + '/calendars/' + calendar.slug + '/') %}
+  <a
+    href="{{ local_href }}"
+    class="{{styles.nav_button_less_pad}}">
+    <i class="fa fa-calendar" aria-hidden="true"></i>
+    <div>{{calendar.name}}</div>
+  </a>
+{% endfor %}