Add machine-readable rip manifest alongside backup-info.txt

[frznn]

Summary: add a machine-readable manifest next to backup-info.txt so backups are easier to inspect, verify, and import programmatically.

Scope ideas:

• write a JSON manifest into the album root on successful runs
• include disc geometry, selected tracks, created artifacts, output formats, metadata source, and verification results
• keep the text manifest for human readability
• make sure the manifest only records files that were actually produced

This is a follow-up item after the current stabilization pass, not a blocker for the current release.