List and audit databases (bur dbs, bur check)

Operator views over discovery, and the safety net that makes auto-discovery trustworthy.

• bur dbs: list the resolved targets with each one's settings.
• bur check: validate every discovered config, warn on any unlabelled postgres container, and exit non-zero on problems, naming the offender. Auth-path probe (a hard criterion): for each DB, confirm the exec auth path actually authenticates (peer/trust or POSTGRES_PASSWORD), so a broken path surfaces here rather than during a real restore.
• Supported images: the official postgres:* and *-alpine images are confirmed. Non-official images (Bitnami non-root, distroless, custom) are supported only if they pass check's probe. Document this in the README (see the docs ticket).

Acceptance

• bur dbs lists every labelled DB with its resolved settings.
• bur check exits non-zero on any unlabelled postgres container or invalid config, naming it.
• check probes the auth path for each discovered DB.