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/static/order/z-a.svg

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="24" height="24" viewBox="0 0 24 24"
+     xmlns="http://www.w3.org/2000/svg" role="img" aria-label="Sort Z to A">
+  <!-- Orange rounded square -->
+  <rect x="1" y="1" width="22" height="22" rx="5" fill="#F97316"/>
+  <!-- A–Z label -->
+  <text x="12" y="12" text-anchor="middle" dominant-baseline="middle"
+        font-family="system-ui, -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Arial, sans-serif"
+        font-size="9" font-weight="700" fill="#FFFFFF">Z-A</text>
+</svg>