security: drop unused MINIO_* env vars from backend service

[gordonmurray]

Closes #27.

What

• Removes `MINIO_ENDPOINT` / `MINIO_ACCESS_KEY` / `MINIO_SECRET_KEY` from the backend service in `docker-compose.yml`. The backend never read them — see `grep -n 'MINIO_' backend/main.py`, no matches.
• Adds a Credential Model section to the README documenting:
  • User query credentials are passed per-request in the API body, applied to a short-lived in-memory DuckDB session, and discarded on connection close. Never env, never disk, never logged.
  • Self-hosters should use Docker secrets (`*_FILE` convention) for any service-level creds they add — links to the Docker docs.
  • The bundled demo MinIO uses plain env vars for the public `cloudfloe` / `cloudfloe123` defaults; that pattern is for demo convenience only and shouldn't be copied for production.
• Tidied the README by removing all horizontal-rule separators — the section headings already provide structure.

Why this scope

The original issue text was inherited from a CLAUDE.md note about credentials leaking via `docker inspect`. On audit that risk only exists in the commercial product (cloudfloe.com), not OSS — OSS user creds were never in env vars. So the right fix here is removing dead surface (the unused vars) and documenting the model, rather than retrofitting Docker secrets onto demo creds that are public anyway.

Verified end-to-end as a first-time user

`docker compose down -v && docker compose up --build`, then via the actual UI in a real (headless) Chrome:

1. Open http://localhost:3000 — page loads, all assets 200, no console errors
2. Pick MinIO, fill table path + demo creds + endpoint, click Test Connection
3. → "Connection successful" + tableInfo panel: Format iceberg-v2 | Rows 37,537 | Files 1 | Last snapshot 2m ago
4. Suggested query auto-loads in the editor
5. Click Run Query → "Query completed in 94ms", 10 rows of real movie data rendered

Side finding (not in this PR)

During the e2e test I caught a real first-time-user bug: switching the Storage Type dropdown to MinIO sets the endpoint placeholder but not the value, so a clicker who doesn't notice the difference gets 400 from AWS. Filed as #35 — easy fix, separate scope.