fix(sync): offline detector wiped device name, auto_sync, and metadata on every transition

setOfflineMode/setOnlineMode only mean to flip sync_enabled, but they
called the full-row UpdateDevice with Go zero values — which wrote
DeviceName: "" and NULLed auto_sync/sync_frequency_minutes/
device_metadata on the device row at every offline->online flip. New
targeted SetDeviceSyncEnabled query touches exactly that column; the
real settings update keeps using UpdateDevice.

Caught as part of the H4 helper-census pass (plan §5.4 erasure list).

internal/database/querier.go

@@ -526,6 +526,7 @@ type Querier interface {
 	SearchNarratorValues(ctx context.Context, arg SearchNarratorValuesParams) ([]SearchNarratorValuesRow, error)
 	SearchSeriesValues(ctx context.Context, arg SearchSeriesValuesParams) ([]SearchSeriesValuesRow, error)
 	SearchTagsValues(ctx context.Context, arg SearchTagsValuesParams) ([]SearchTagsValuesRow, error)
+	SetDeviceSyncEnabled(ctx context.Context, arg SetDeviceSyncEnabledParams) (Devices, error)
 	// Library Visibility queries
 	SetLibraryVisibility(ctx context.Context, arg SetLibraryVisibilityParams) (LibraryVisibility, error)
 	// Set system config

internal/database/queries.sql.go

@@ -13116,6 +13116,42 @@ func (q *Queries) SearchTagsValues(ctx context.Context, arg SearchTagsValuesPara
 	return items, nil
 }
 
+const SetDeviceSyncEnabled = `-- name: SetDeviceSyncEnabled :one
+UPDATE devices
+SET
+    sync_enabled = $2,
+    updated_at = NOW()
+WHERE id = $1
+RETURNING id, user_id, device_name, device_type, device_identifier, auth_token, last_sync, last_seen, sync_enabled, auto_sync, sync_frequency_minutes, device_metadata, created_at, updated_at
+`
+
+type SetDeviceSyncEnabledParams struct {
+	ID          pgtype.UUID `db:"id" json:"id"`
+	SyncEnabled pgtype.Bool `db:"sync_enabled" json:"sync_enabled"`
+}
+
+func (q *Queries) SetDeviceSyncEnabled(ctx context.Context, arg SetDeviceSyncEnabledParams) (Devices, error) {
+	row := q.db.QueryRow(ctx, SetDeviceSyncEnabled, arg.ID, arg.SyncEnabled)
+	var i Devices
+	err := row.Scan(
+		&i.ID,
+		&i.UserID,
+		&i.DeviceName,
+		&i.DeviceType,
+		&i.DeviceIdentifier,
+		&i.AuthToken,
+		&i.LastSync,
+		&i.LastSeen,
+		&i.SyncEnabled,
+		&i.AutoSync,
+		&i.SyncFrequencyMinutes,
+		&i.DeviceMetadata,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+	)
+	return i, err
+}
+
 const SetLibraryVisibility = `-- name: SetLibraryVisibility :one
 INSERT INTO library_visibility (user_id, library_id, is_visible)
 VALUES ($1, $2, $3)

internal/database/queries/queries.sql

@@ -1669,6 +1669,14 @@ SET
 WHERE id = $1
 RETURNING *;
 
+-- name: SetDeviceSyncEnabled :one
+UPDATE devices
+SET
+    sync_enabled = $2,
+    updated_at = NOW()
+WHERE id = $1
+RETURNING *;
+
 -- name: UpdateDeviceLastSeen :one
 UPDATE devices
 SET