Import iPhoto library into Apple Photos (local library organization)

[cdevers]

Goal

Import the legacy iPhoto library into the active Apple Photos library so that those photos are available locally for editing, organization, and future scanning by `bp scan`.

Relationship to other issues

By the time this is done, #229 + #230 will have handled the Flickr gap — every photo in the legacy library will either already have a Flickr ID or will have been uploaded. This issue is therefore purely about local library organization: getting the files into Photos.app for editing, keyword management, and long-term storage in the active library.

Blocker

iCloud storage. Importing the full legacy library into Apple Photos will trigger iCloud sync unless iCloud Photos is disabled for the import duration (risky) or storage is expanded. Until the storage question is resolved, no code is needed.

Options:

• Upgrade iCloud storage tier
• Import selectively by year, letting storage catch up
• Import with iCloud Photos paused, then re-enable once quota is addressed

What BP needs to do after import

Once photos are in Apple Photos and iCloud has synced:

1. Run `bp scan` — discovers newly imported photos and populates `uuid` on previously Flickr-only records (linking them to their Apple Photos counterpart via timestamp matching / `link_orphans`)
2. Normal metadata sync pipeline takes over — generates proposals for tag/title/description drift
3. Optional: extract and migrate iPhoto keywords that didn't survive the Photos import cleanly

iPhoto keyword migration (M-4 from original #12)

iPhoto keywords were not always cleanly migrated when Photos imported the library. BP could read iPhoto's SQLite database directly (pre-import) to extract keyword associations and write them as Photos keywords post-import. Low priority; assess after the import.

Notes

• `link_orphans` and the timestamp-rounding fix already handle the bulk of uuid↔flickr_id linking automatically once UUIDs are populated by scan.
• The 109k Flickr-only photos remain accessible in the reviewer with CDN thumbnails throughout.

Replaces #12. Depends on #229, #230.