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)

federation/models/__init__.py

@@ -0,0 +1,9 @@
+"""Re-export federation models from shared.models."""
+from shared.models.federation import (  # noqa: F401
+    ActorProfile,
+    APActivity,
+    APFollower,
+    APInboxItem,
+    APAnchor,
+    IPFSPin,
+)