Threat model T6: Fernet token-encryption key had no rotation path. Rotating
TOKEN_ENCRYPTION_KEY immediately invalidated every github_token_encrypted
value in thread metadata, forcing re-auth or bot-token re-resolution.
Switch to cryptography.fernet.MultiFernet and parse TOKEN_ENCRYPTION_KEY as a
comma- or newline-separated ordered list (most-recent-first). New writes
encrypt under the first key; reads try every key. Deployers can prepend a new
key, let active threads roll over, then drop the old key.
Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Johannes du Plessis <51395795+johannes117@users.noreply.github.com>
