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)

blog/bp/fragments/__init__.py

@@ -0,0 +1 @@
+from .routes import register as register_fragments

blog/bp/fragments/routes.py

@@ -0,0 +1,52 @@
+"""Blog app fragment endpoints.
+
+Exposes HTML fragments at ``/internal/fragments/<type>`` for consumption
+by other coop apps via the fragment client.
+"""
+
+from __future__ import annotations
+
+from quart import Blueprint, Response, g, render_template, request
+
+from shared.infrastructure.fragments import FRAGMENT_HEADER
+from shared.services.navigation import get_navigation_tree
+
+
+def register():
+    bp = Blueprint("fragments", __name__, url_prefix="/internal/fragments")
+
+    # Registry of fragment handlers: type -> async callable returning HTML str
+    _handlers: dict[str, object] = {}
+
+    @bp.before_request
+    async def _require_fragment_header():
+        if not request.headers.get(FRAGMENT_HEADER):
+            return Response("", status=403)
+
+    @bp.get("/<fragment_type>")
+    async def get_fragment(fragment_type: str):
+        handler = _handlers.get(fragment_type)
+        if handler is None:
+            return Response("", status=200, content_type="text/html")
+        html = await handler()
+        return Response(html, status=200, content_type="text/html")
+
+    # --- nav-tree fragment ---
+    async def _nav_tree_handler():
+        app_name = request.args.get("app_name", "")
+        path = request.args.get("path", "/")
+        first_seg = path.strip("/").split("/")[0]
+        menu_items = await get_navigation_tree(g.s)
+        return await render_template(
+            "fragments/nav_tree.html",
+            menu_items=menu_items,
+            frag_app_name=app_name,
+            frag_first_seg=first_seg,
+        )
+
+    _handlers["nav-tree"] = _nav_tree_handler
+
+    # Store handlers dict on blueprint so app code can register handlers
+    bp._fragment_handlers = _handlers
+
+    return bp