Add Kopia backup stack with Backblaze B2 offsite target.

Protects /config, selective labdata, and nightly DB dumps; documents homelab improvement plan briefs.

stacks/backup/snapshot-paths.txt

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+# Kopia snapshot roots (one path per line). Edit when scope changes.
+/backup/config
+/backup/dumps
+/backup/labdata/gitea/repositories
+/backup/labdata/gitea/lfs
+/backup/labdata/famapp/minio
+/backup/labdata/resume/minio
+/backup/labdata/plane/minio
+/backup/labdata/romhacks/metadata
+/backup/labdata/romhacks/library
+/backup/labdata/romm/assets