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/contracts/__init__.py

@@ -0,0 +1,31 @@
+"""Typed contracts (DTOs + Protocols) for cross-domain service interfaces."""
+
+from .dtos import (
+    PostDTO,
+    CalendarDTO,
+    CalendarEntryDTO,
+    MarketPlaceDTO,
+    ProductDTO,
+    CartItemDTO,
+    CartSummaryDTO,
+)
+from .protocols import (
+    BlogService,
+    CalendarService,
+    MarketService,
+    CartService,
+)
+
+__all__ = [
+    "PostDTO",
+    "CalendarDTO",
+    "CalendarEntryDTO",
+    "MarketPlaceDTO",
+    "ProductDTO",
+    "CartItemDTO",
+    "CartSummaryDTO",
+    "BlogService",
+    "CalendarService",
+    "MarketService",
+    "CartService",
+]