refactor: drop VITE_API_BASE_URL, always use relative /api

[a-mandala]

Summary

The frontend no longer reads VITE_API_BASE_URL. It always targets the relative /api path.

• web/src/lib/env.ts: hardcode API_BASE_URL = "/api"
• web/vite.config.ts: dev proxy forwards /api to http://127.0.0.1:3000 so npm run dev works against cargo run with zero configuration
• Remove the variable (and now-pointless build arg plumbing) from backend/Dockerfile, docker-compose.build.yml, CI workflow, and delete web/.env.example
• Update both READMEs accordingly

Rationale

The backend serves the bundled frontend on the same origin and exposes its API under /api, so build-time URL configurability was dead weight that forced a rebuild per environment. Hosting the API on a separate origin is still possible with a reverse proxy forwarding /api.

Validation

• npm run lint: OK
• npm run typecheck: OK
• npm test: 84/84 passing
• npm run build: OK
• Docker image build not exercised locally; CI will cover it.