Add encrypted secure backup export

[alceops]

Summary

• Adds authenticated POST /auth/me/backup/export for encrypted, user-scoped backups covering categories, expenses, recurring expenses, bills, and reminders.
• Uses passphrase-derived Fernet authenticated encryption (random salt + PBKDF2-HMAC-SHA256) and records an encrypted_backup_exported audit event without returning/logging plaintext.
• Adds a Redis in-memory fallback so auth/cache flows continue in local/free-tier environments when Redis is unavailable.
• Documents the endpoint and adds regression tests for encrypted export, weak passphrase rejection, wrong-passphrase rejection, and existing auth behavior.

Verification

• PYTHONPATH=. ../../.venv/bin/pytest tests -> 25 passed

Closes #126