Summary

• exclude mempool's persistent backend cache from umbrelOS backups
• exclude the chain-derived MariaDB explorer index from umbrelOS backups
• bump the package to 3.3.1-hotfix-2 so existing installations receive the change
• explain restore behavior in the release notes

Why

Both excluded paths are derived from Bitcoin Node and Electrs data. Backing them up duplicates rebuildable chain/index data and can consume substantial backup storage.

After a restore, mempool rebuilds these paths from Bitcoin Node and Electrs. Locally accumulated explorer statistics may take time to repopulate. Wallet and node configuration remain owned and backed up by their respective apps.

Verification

• official image-aware app linter: 0 errors
• exact assertion confirms backupIgnore matches ${APP_DATA_DIR}/data:/backend/cache and ${APP_DATA_DIR}/mysql/data:/var/lib/mysql
• package directory skeletons remain present for fresh installs/restores
• git diff --check passes
• changed-file scope is limited to mempool/umbrel-app.yml

Existing warning

The linter reports that the mutable mariadb:10.5.12 tag no longer resolves to the digest already pinned by the package. This PR intentionally preserves the existing immutable digest and does not update any images.