Fix local dev routing and unify slug availability checks

[alankey-dev]

Summary

• add a shared slug assessment path so /subdomains/check and /subdomains/purchase enforce the same domain, reserved-word, profanity, blocklist, and taken checks
• fix the dashboard availability flow so it uses backend reasons, caches by slug plus domain, and stops reporting hello as available before failing on create
• split local Docker/Traefik behaviour from production, use *.localhost for local edge routing, allow private/LAN origins in local mode, and isolate Docker E2E with a dedicated compose project name

Verification

• . .venv312/bin/activate && pytest api/tests router/tests -q
• docker compose -f docker-compose.yml -f docker-compose.local.yml up -d --build
• curl -sf http://api.localhost/health
• curl -sf http://hostapp.localhost/health
• browser check via Playwright: hello shows as available and creates successfully; hell shows the profanity rejection inline
• docker compose -f docker-compose.e2e.yml up --build --abort-on-container-exit