Add legal disclaimer, indemnification clause, and acceptable use policy

- README: visible disclaimer in EN + ES sections
- LEGAL_DISCLAIMER.md: full legal terms (GDPR, EU AI Act, indemnification, acceptable use)
- CONTRIBUTING.md: explicit "what we do NOT accept" section (scraping, auto-submit, personal data)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>