v0.4.12: fix the CI lint failure so the image builds and publishes

Fixes the CI lint failure introduced by v0.4.11 so the image builds and publishes.

- v0.4.11 added make lint to CI for the first time, which surfaced a pre-existing flake8
  E501: a 103-character line in src/mailboxd/server.py. The lint job failed and the image
  build and publish steps, which depend on it, were skipped. Wrapped the line so flake8
  passes. No behavior change.