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/models/__init__.py

@@ -0,0 +1,2 @@
+from .order import Order, OrderItem
+from .page_config import PageConfig

cart/models/order.py

@@ -0,0 +1 @@
+from shared.models.order import Order, OrderItem  # noqa: F401

cart/models/page_config.py

@@ -0,0 +1 @@
+from shared.models.page_config import PageConfig  # noqa: F401