Harden credential encryption key derivation (PBKDF2 with versioned ciphertext)

Context

open_notebook/utils/encryption.py derives the Fernet key from OPEN_NOTEBOOK_ENCRYPTION_KEY with a bare, unsalted hashlib.sha256(key.encode()). There are no tests for the module. #1020 proposed PBKDF2 but went stale (branch stacked on since-merged PRs, author inactive since July), so this issue tracks the work for a fresh PR off current main. Credit to @kwp3 for the original proposal.

Scope

• Derive the key with PBKDF2-HMAC-SHA256 (600k iterations, fixed app salt) instead of plain SHA-256.
• Prefix new ciphertext with a version marker (e.g. pbkdf2v1:) and keep a legacy decrypt path for unprefixed values.
• One-shot re-encryption command (commands/) that rewrites existing Credential.api_key / ProviderConfig.api_key values, fail-closed per record. Lazy re-encrypt on save is not enough: API keys are set once and would never upgrade.
• tests/test_encryption.py: round-trip, legacy fallback, marker handling, re-encryption command.
• Docs: security.md guidance on high-entropy keys; CHANGELOG entry with a downgrade warning (older versions can't read pbkdf2v1: values).
• Short decision record in docs/7-DEVELOPMENT/decisions/.

Supersedes #1020.