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)

cart/bp/cart/services/__init__.py

@@ -0,0 +1,13 @@
+from .get_cart import get_cart
+from .identity import current_cart_identity
+from .total import total
+from .clear_cart_for_order import clear_cart_for_order
+from .calendar_cart import get_calendar_cart_entries, calendar_total, get_ticket_cart_entries, ticket_total
+from .check_sumup_status import check_sumup_status
+from .page_cart import (
+    get_cart_for_page,
+    get_calendar_entries_for_page,
+    get_tickets_for_page,
+    get_cart_grouped_by_page,
+)
+