fix(images): key favorites by the real root and case-normalized path

Folder names are not unique across gallery roots, so two roots shown as
"output" shared each other's stars. Favorites are now keyed by the
root's real, case-normalized path (server-side only) and a
case-normalized relative path, so a Windows case-only rename keeps its
star while distinct Linux files stay distinct. Toggles look the file up
in a per-scan dictionary instead of walking the listing.

tests/test_database_backup.py

@@ -488,7 +488,7 @@ class TestRestoreFromFile(BackupTestCase):
         with closing(sqlite3.connect(str(backups[0]))) as conn:
             self.assertEqual(
                 conn.execute(
-                    "SELECT root_label, relative_path FROM image_favorites"
+                    "SELECT root_key, relative_path FROM image_favorites"
                 ).fetchall(),
                 [("output", "live.png")],
             )