Back up every database

Go from one database to multiple, with a second copy on Cloudflare R2.

• Discover opt-in databases from Docker labels (bur.enable=true), reading each one's settings from bur.* labels on the running container.
• Back them up in one loop with per-DB failure isolation - one DB failing never aborts the rest.
• Track health (a two-boolean state file: dumped-local + pushed-offsite), prune locally to the last N, and let each project set its own cadence so a single cron (bur backup --due) serves them all.
• Copy every backup to Cloudflare R2, restorable from R2 alone, plus a path to restore to a local dev container.

Done when: bur backup backs up every labelled container; one failing doesn't stop the others; each backup also copied to R2; bur status --json reports per-DB health; and a backup can be restored with the local copy gone.