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)

shared/events/handlers/container_handlers.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from shared.events import register_activity_handler
+from shared.models.federation import APActivity
+from shared.services.navigation import rebuild_navigation
+
+
+async def on_child_attached(activity: APActivity, session: AsyncSession) -> None:
+    await rebuild_navigation(session)
+
+
+async def on_child_detached(activity: APActivity, session: AsyncSession) -> None:
+    await rebuild_navigation(session)
+
+
+register_activity_handler("Add", on_child_attached, object_type="rose:ContainerRelation")
+register_activity_handler("Remove", on_child_detached, object_type="rose:ContainerRelation")