feat: per-library destination subdirectories (photos.library_destinations)

[epheterson]

Summary

New optional config block photos.library_destinations maps each iCloud
library to a subdirectory of photos.destination:

```yaml
photos:
destination: photos
library_destinations:
PrimarySync: personal
SharedLibrary: shared
```

Personal photos then land in <photos.destination>/personal/, Shared
Library photos in <photos.destination>/shared/. When library_destinations
is unset (the default), all libraries share the single photos.destination
tree — preserving the historical mandarons/icloud-docker behaviour, no
breaking change.

Motivation

Common request from users who want their iCloud Shared Library kept
separate from their personal photos on disk (for separate Plex libraries,
different sharing/backup policies, etc.). Currently the only way to get
this is to run two mandarons/icloud-docker containers with different
libraries: filters — which means two auth events and two cookie stores
for the same Apple ID.

Implementation

• New config_parser.get_photos_library_destinations(config) returns a
  dict mapping library name → subdir (empty dict when unset).
• New sync_photos._library_destination(base, library, mapping) helper
  resolves the per-library destination and creates the subdir.
• Threaded through sync_photos.sync_photos, _sync_all_photos_first_for_hardlinks,
  and _sync_albums_by_configuration as a kwarg with None default
  (backward-compatible).
• Obsolete-file cleanup walks each per-library subdir independently when
  library_destinations is set, falling back to the legacy
  single-destination walk otherwise.

Tests

10 new tests in tests/test_library_destinations.py:

• Config helper: returns {} when unset / when not a dict / coerces
  non-string keys+values to str, returns mapping when configured.
• _library_destination helper: returns base when no mapping, returns
  base when library not in mapping, joins subdir + creates dir, handles
  nested subdirs, None mapping is safe.

[Copilot]

Pull request overview

Adds an optional photos.library_destinations config block that maps each iCloud library (e.g. PrimarySync, SharedLibrary/SharedSync-…) to a subdirectory under photos.destination, so personal and shared library content can be split on disk without running two containers. Behavior is unchanged when the new block is omitted.

Changes:

• New config_parser.get_photos_library_destinations() returning a dict[str, str] (or {} when unset/invalid), with key/value coercion to str.
• New _library_destination() helper in sync_photos.py that resolves per-library subdirs (exact match → SharedLibrary alias for SharedSync-… zones → fallthrough), creating the directory as needed, and threads it through _sync_all_photos_first_for_hardlinks and _sync_albums_by_configuration.
• Obsolete-file cleanup now walks each configured library's subdir individually when the mapping is set, falling back to the legacy single-destination walk otherwise. Adds tests/test_library_destinations.py covering the helper and per-library cleanup.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/config_parser.py	Adds get_photos_library_destinations returning a normalized dict[str, str] mapping (or {}).
src/sync_photos.py	Plumbs library_destinations through sync entrypoints, adds _library_destination resolver, and per-library cleanup loop.
tests/test_library_destinations.py	Unit tests for the config helper, _library_destination rules (incl. SharedLibrary alias), and per-library remove_obsolete_files invocation.