* fix: preserve created_at when moving songs between libraries (#5050)
When songs are moved between libraries, their creation date was being
reset to the current time, causing them to incorrectly appear in
"Recently Added". Three changes fix this:
1. Add hash:"ignore" to AlbumID in MediaFile struct so that Equals()
works for cross-library moves (AlbumID includes library prefix,
making hashes always differ between libraries)
2. Preserve album created_at in moveMatched() via CopyAttributes,
matching the pattern already used in persistAlbum() for
within-library album ID changes
3. Only set CreatedAt in Put() when it's zero (new files), and
explicitly copy missing.CreatedAt to the target in moveMatched()
as defense-in-depth for the INSERT code path
* test: add regression tests for created_at preservation (#5050)
Add tests covering the three aspects of the fix:
- Scanner: moveMatched preserves missing track's created_at
- Scanner: CopyAttributes called for album created_at on album change
- Scanner: CopyAttributes not called when album ID stays the same
- Persistence: Put sets CreatedAt to now for new files with zero value
- Persistence: Put preserves non-zero CreatedAt on insert
- Persistence: Put does not reset CreatedAt on update
Also adds CopyAttributes to MockAlbumRepo for test support.
* test: verify album created_at is updated in cross-library move test (#5050)
Added end-to-end assertion in the cross-library move test to verify that
the new album's CreatedAt field is actually set to the original value after
CopyAttributes runs, not just that the method was called. This strengthens
the test by confirming the mock correctly propagates the timestamp.
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