chore(backend): env example, dockerignore, regenerate lockfile

  * backend/.env.example                - full set of env vars including
                                          AUTH_PROVIDER block (local /
                                          supabase / entra), Azure
                                          storage block, OpenAI/AOAI,
                                          download-signing secret,
                                          observability connection
                                          string.  Sanitized - concrete
                                          identifiers replaced with
                                          placeholders.

  * backend/.dockerignore               - standard Node ignore set so
                                          docker build doesn't blow up
                                          the image with node_modules.

  * backend/package-lock.json           - regenerated to match the
                                          dependency set added across
                                          earlier commits (azure SDKs,
                                          pg / node-pg-migrate, openai,
                                          cookie-parser).