Add read-only iCloud Photos support to the existing iclouddrive
backend via `service = photos` config option.
Also includes auth improvements on top of #9209's SRP authentication.
**Photos features:**
- 3-level hierarchy: libraries (Personal + Shared Photo Library) →
albums → photos/videos
- server-side smart albums (All Photos, Videos, Favorites,
Screenshots, Live, Bursts, Panoramas, Slo-mo, Time-lapse, Portrait,
Long Exposure, Animated, Hidden, Recently Deleted)
- User-created albums and nested album folders
- Live Photo `.MOV` companions as first-class entries
- Edited photo versions (`-edited` suffix) and RAW alternatives
- Duplicate filename dedup for camera counter wrap collisions
- Parallel cold listing for large albums
- Delta sync via CloudKit `changes/zone` - warm listings near-instant from disk cache
- Disk cache (libraries, albums, photos) with atomic writes for crash safety
- `ChangeNotify` support for FUSE mounts via `changes/zone` polling
- `ListR` support for `--fast-list` and recursive operations
- `--metadata` support - width, height, added-time, favorite, hidden
- Fresh download URLs per file - no stale URL failures on long copies
- FUSE mount documentation with recommended flags
**Auth improvements over #9209:**
- SMS 2FA fallback for users without trusted Apple devices
- Explicit push notification request - fixes iOS/macOS 26.4+ where 409
no longer auto-pushes
- Thread safety for concurrent FUSE callers (mutexes on session and client state)
- Session endpoint caching - skips ~5s `/validate` round-trip on warm start
- `Disconnect` support - clears auth state + disk cache
- PCS cookie support for Advanced Data Protection accounts, including
trusted-device approval for PCS cookies
Built on @coughlanio's Photos PoC (Closes#8734) and @mikegillan's SRP auth (#9209).
Fixes#7982
Co-authored-by: Chris Coughlan <chris@coughlan.io>
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