feat: add Docker and Docker Compose support for self-hosting

From the PR description

Summary

• Adds backend/Dockerfile and frontend/Dockerfile for both services
• Adds docker-compose.yml at the repo root to wire them together
• Adds .env.example documenting the three build-time vars needed by Compose
• Sets output: "standalone" in next.config.ts for a smaller frontend runtime image

Why

Makes self-hosting straightforward - the full stack can be built and started with a single command:

docker compose up --build

No platform-specific tooling required. The deployment still uses the existing Supabase and Cloudflare R2 infrastructure - this is purely about packaging and running the app, not replacing any services.

Changes

• backend/Dockerfile: multi-stage build; installs LibreOffice in the runner stage (required by libreoffice-convert), compiles TypeScript and runs node dist/index.js
• frontend/Dockerfile: multi-stage build; NEXT_PUBLIC_* vars passed as build args (baked in at build time by Next.js), copies only the standalone output for a minimal image
• docker-compose.yml: exposes backend on 3001, frontend on 3000; frontend depends on backend; both load env from their respective .env files
• .env.example: documents the root-level build args for Compose
• frontend/next.config.ts: adds output: "standalone" - required for the optimized Docker image, no effect on the existing Cloudflare deployment path

Testing

Built and ran locally with both containers healthy:

• Backend: http://localhost:3001/health returns {"ok":true}
• Frontend: http://localhost:3000 serves the app