v4: stacks, hosts and databases

Spec: SPEC. Build log: PROGRESS. Draft: DRAFT_SPEC. Merged in PR #27.

• Stack composition from a versioned catalog, with compatibility rules in the backend, replacing the v2 presets.
• Specialist agents with pack skills.
• GitLab and Bitbucket Cloud: sign-in, connections, repo picker, merge requests, CI status, releases and import.
• Deployment platforms: Vercel, Netlify, Cloudflare, GitHub Pages, Render and Railway, with an environment inventory and an origin model.
• Managed databases: Supabase, Render Postgres and Railway Postgres, per environment.
• LLM choice per deployment, with streaming and continued answers.