start_playlist_sync validated the resolved mode with 'if sync_mode not in
(replace, append): sync_mode = replace' — a pre-existing clamp two lines below
the config read I added, which silently downgraded a configured 'reconcile' to
'replace'. So config=reconcile resolved correctly then got clobbered, and every
sync ran replace regardless of the setting (and incognito didn't help — it's
backend, not cache).
Replace the hand-rolled clamp with a pure normalize_sync_mode(requested,
configured) helper (VALID_SYNC_MODES includes reconcile) so the resolution is
testable and can't silently drop a mode again. Regression tests cover
reconcile-from-config, request-overrides-config, and unknown->replace.
65c9948c78