fix(settings): register player.dolby_vision_enabled and player.seek_cache_enabled

Keys absent from settingsRegistry resolve through the user-scope path
and return an empty effective_value, which Apple clients interpreted as
false — flipping these default-ON toggles off on first sync. Register
both device-scoped keys with default "true" so defaults resolve
correctly and device overrides round-trip. seek_cache_enabled had been
syncing unregistered since it shipped; dolby_vision_enabled is new
(Apple client Dolby Vision toggle, silo-apple e9bd775).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/api/handlers/settings.go

@@ -231,11 +231,21 @@ var settingsRegistry = map[string]settingSpec{
 		DefaultValue: "true",
 		Validate:     validateBoolSetting("player.hdr_enabled"),
 	},
+	"player.dolby_vision_enabled": {
+		Scope:        scopeDevice,
+		DefaultValue: "true",
+		Validate:     validateBoolSetting("player.dolby_vision_enabled"),
+	},
 	"player.dv_profile7_hdr10_fallback": {
 		Scope:        scopeDevice,
 		DefaultValue: "false",
 		Validate:     validateBoolSetting("player.dv_profile7_hdr10_fallback"),
 	},
+	"player.seek_cache_enabled": {
+		Scope:        scopeDevice,
+		DefaultValue: "true",
+		Validate:     validateBoolSetting("player.seek_cache_enabled"),
+	},
 	"player.playback_speed": {
 		Scope:        scopeDevice,
 		DefaultValue: "1",