fix(devices): device deletion detaches reading history instead of 500ing

Deleting any device whose stats rows still referenced it failed with
500: reading_history.device_id is a NO-ACTION FK by design (the rows
are real captured reading stats — Users reads them), so a blind DELETE
violated the constraint. Surfaced by the Android app's new logout
deregister call, but the admin Devices page Deletes hit the same stone.

Fix: DetachReadingHistoryFromDevice — NULL the device association on
the user's history rows before the device DELETE (single handler
pass). device_id is nullable; the stats survive, the device frees its
identifier. sqlc regenerated.

internal/database/queries.sql.go

@@ -2362,6 +2362,20 @@ func (q *Queries) DeleteUserSystemCollection(ctx context.Context, arg DeleteUser
 	return err
 }
 
+const DetachReadingHistoryFromDevice = `-- name: DetachReadingHistoryFromDevice :exec
+UPDATE reading_history SET device_id = NULL WHERE device_id = $1
+`
+
+// Device deletion (deregister on logout): detach the device's reading
+// history instead of deleting it — these rows are real captured stats
+// (the Users section reads them); reading_history.device_id is NO ACTION
+// so a blind device delete 500s once any history anchors to the device.
+// device_id is nullable: NULL keeps the session, loses the association.
+func (q *Queries) DetachReadingHistoryFromDevice(ctx context.Context, deviceID pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, DetachReadingHistoryFromDevice, deviceID)
+	return err
+}
+
 const DeviceExistsForUser = `-- name: DeviceExistsForUser :one
 SELECT EXISTS(
     SELECT 1 FROM devices