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/browser/app/__init__.py

@@ -0,0 +1,12 @@
+# The monolith has been split into three apps (apps/blog, apps/market, apps/cart).
+# This package remains for shared infrastructure modules (middleware, redis_cacher,
+# csrf, errors, authz, filters, utils, bp/*).
+#
+# To run individual apps:
+#   hypercorn apps.blog.app:app --bind 0.0.0.0:8000
+#   hypercorn apps.market.app:app --bind 0.0.0.0:8001
+#   hypercorn apps.cart.app:app --bind 0.0.0.0:8002
+#
+# Legacy single-process:
+#   hypercorn suma_browser.app.app:app --bind 0.0.0.0:8000
+#   (runs the old monolith from app.py, which still works)