The mirror_playlist fix only assigns stable ids to newly-imported playlists, so a
user with an existing file-import playlist would still have empty-id rows (and dead
Find & Add matches) until a manual re-import. Add an idempotent startup backfill that
assigns the SAME stable id a fresh import would to any mirrored track missing one —
so existing matches start sticking with no re-import. Runs once per db/process (the
init is guarded), only touches empty-id rows (no-op afterward), native ids untouched.
Tests: backfill fills empty ids with the exact fresh-import id, is idempotent (2nd
run = 0), and leaves native ids alone.
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