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/queries.sql

@@ -2809,6 +2809,14 @@ SET
     updated_at = NOW()
 WHERE id = $1;
 
+-- name: DetachReadingHistoryFromDevice :exec
+-- 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.
+UPDATE reading_history SET device_id = NULL WHERE device_id = $1;
+
 -- name: DeleteDevice :exec
 DELETE FROM devices WHERE id = $1;