feat: always bind to 0.0.0.0

Reachability is controlled by the host firewall / compose port
publishing, not the bind address. Backend, compose, and docs all
now default to 0.0.0.0 so the API is reachable from the frontend
container, the host, and the LAN without per-env overrides.

Files:
- backend/src/cyclone/__main__.py: default host = 0.0.0.0
- docker-compose.yml: refresh the comment to match the new posture
- CLAUDE.md / README.md / docs/ARCHITECTURE.md / docs/REQUIREMENTS.md:
  reframe the bind note accordingly

README.md

@@ -30,7 +30,7 @@ Two terminals:
 # Terminal 1 — backend
 cd backend
 .venv/bin/python -m cyclone serve
-# (defaults to 127.0.0.1:8000; override with CYCLONE_PORT=...; reload with CYCLONE_RELOAD=1)
+# (defaults to 0.0.0.0:8000; override with CYCLONE_HOST / CYCLONE_PORT / CYCLONE_RELOAD=1)
 
 # Terminal 2 — frontend
 npm run dev