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)

market/scrape/build_snapshot/tools/candidate_subs.py

@@ -0,0 +1,14 @@
+from typing import Dict, Set
+
+def candidate_subs(nav: Dict[str, Dict])-> Set[str]:
+    nav_sub_candidates: Set[str] = set()
+    for label, data in (nav.get("cats") or {}).items():
+        top_slug = (data or {}).get("slug")
+        if not top_slug:
+            continue
+        for s in (data.get("subs") or []):
+            sub_slug = (s.get("slug") or "").strip()
+            if sub_slug:
+                nav_sub_candidates.add(f"/{top_slug}/{sub_slug}")
+    return nav_sub_candidates
+