#845 tests: cover the verification_status migration backfill

Locks the one-time backfill that derives verification_status for pre-column
library_history rows from acoustid_result: pass->verified, skip->unverified,
fail->force_imported; never overwrites an existing status (NULL-only); leaves
no-acoustid rows NULL; idempotent across repeated inits. Exercises the real
_initialize_database path (clears the per-process init guard to re-trigger it).

tests/test_verification_status_backfill.py

@@ -0,0 +1,53 @@
+"""#845: the migration backfills verification_status for library_history rows
+written before the column existed, from the acoustid_result they recorded. Must
+map correctly, never overwrite an existing status, leave no-acoustid rows alone,
+and be idempotent (it runs on every fresh DB init).
+
+The backfill runs inside _initialize_database, which is guarded to run once per
+path per process — so the tests clear that guard to re-trigger it on the seeded
+rows (exercising the real production code path, not a copy of the SQL)."""
+
+from __future__ import annotations
+
+import database.music_database as mdb
+from database.music_database import MusicDatabase
+
+
+def _reinit(path):
+    """Force _initialize_database (incl. the backfill) to run again on `path`."""
+    mdb._database_initialized_paths.clear()
+    return MusicDatabase(path)
+
+
+def _statuses(db):
+    with db._get_connection() as conn:
+        return {r['title']: r['verification_status']
+                for r in conn.execute("SELECT title, verification_status FROM library_history")}
+
+
+def test_backfill_maps_and_preserves(tmp_path):
+    p = str(tmp_path / "m.db")
+    db = MusicDatabase(p)
+    db.add_library_history_entry('import', 'A', acoustid_result='pass')
+    db.add_library_history_entry('import', 'B', acoustid_result='skip')
+    db.add_library_history_entry('import', 'C', acoustid_result='fail')
+    db.add_library_history_entry('import', 'D', acoustid_result='pass',
+                                 verification_status='human_verified')  # pre-set, must NOT change
+    db.add_library_history_entry('import', 'E')                         # no acoustid → stays NULL
+
+    _reinit(p)   # run the backfill migration over the seeded rows
+
+    s = _statuses(db)
+    assert s['A'] == 'verified'
+    assert s['B'] == 'unverified'
+    assert s['C'] == 'force_imported'
+    assert s['D'] == 'human_verified'   # existing status preserved (NULL-only)
+    assert s['E'] is None               # no acoustid_result → untouched
+
+
+def test_backfill_is_idempotent(tmp_path):
+    p = str(tmp_path / "m2.db")
+    db = MusicDatabase(p)
+    db.add_library_history_entry('import', 'A', acoustid_result='pass')
+    _reinit(p); _reinit(p)              # run the migration two more times
+    assert _statuses(db)['A'] == 'verified'