Add incremental encrypted Drive image backups

[catherineluse]

Background

Image content is currently included in the encrypted backup payload. This preserves parity, but re-uploads unchanged image bytes and makes one large backup object.

Scope

• Store each image as a separately encrypted Drive object keyed by image ID or content hash.
• Add a versioned manifest to the main backup.
• Upload only new or changed image objects.
• Define atomic backup-generation and cleanup semantics so interrupted backups cannot corrupt the last good restore point.
• Restore images lazily or in bounded batches, with clear missing-object recovery behavior.

Acceptance criteria

• Unchanged images are not re-uploaded.
• Interrupted backup/restore leaves the previous complete generation usable.
• Encryption remains client-side and password-compatible.
• Legacy single-payload backups remain restorable.
• Tests cover incremental upload, deletion, interruption, and missing objects.