Drift item #7: remove the password-manager pattern (derive_password, derive_password_string, site_password_path) — not relevant to an RPC system's vault. Removed methods, path function, doc-table row, all tests, and the now-unused base64 URL_SAFE_NO_PAD import. Refs: docs/architecture/crates/vault/README.md drift #7 Implements: ADR-025 (resolves C9)