The method mark_not_downloaded() was misleading — it doesn't negate
'downloaded' history (the model was indeed downloaded before), but
rather sets is_deleted_override = 1 to indicate the version was
downloaded and subsequently deleted. This flag allows re-download when
the 'skip previously downloaded' setting is enabled.
Rename to mark_as_deleted() to accurately reflect its semantics.
Replace _SYNC_KEYS (37 keys) with _NO_SYNC_KEYS (5 keys) in SettingsHandler.
New settings automatically sync to frontend unless explicitly excluded.
Changes:
- SettingsHandler now syncs all settings except those in _NO_SYNC_KEYS
- Added keys() method to SettingsManager for iteration
- Updated tests to use new behavior
Benefits:
- No more missing keys when adding new settings
- Reduced maintenance burden
- Explicit exclusions for sensitive/internal settings only
Fixes: #86
