Allow application errors to be pinned to prevent automatic deletion

[sleberknight]

Allow specific application errors to be pinned so that they are excluded from automatic cleanup. Pinned errors persist until they are explicitly resolved or unpinned.

See discussion: #314

[sleberknight]

The 5.0.0 design doc generated by Claude Code is currently on a separate branch: https://github.com/kiwiproject/dropwizard-application-errors/blob/64461e29ea1b4bffc7cc4d19b09cb57d342f9582/next-major-version-design.md