Symmetric to the post-download reconcile (which handles ADDITIONS): when a
playlist's membership is re-synced (the mirror step — scheduled refresh or the
manual mirror endpoint), rebuild its folder from current membership WITH prune
IF it's organize-by-playlist. So a track that just LEFT the playlist has its
symlink cleaned up the instant membership changes, not only on the next download.
Factored a shared _rebuild_one_from_db (used by the manual 'Rebuild' button and
the mirror hook) + rebuild_mirrored_playlist_if_organized. Gated to organized
playlists, non-fatal at both mirror call sites.
Now the invariant 'folder = the playlist's current owned members' holds on every
change: additions caught at download, removals caught at mirror. 2 new tests
(removed track pruned; non-organized skipped). 985 + 277 tests pass.
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