Add mitigation with task and mapping import

Signed-off-by: gearnode <bryan@frimin.fr>

pkg/coredata/migrations/20250401T175300Z.sql

@@ -0,0 +1,2 @@
+ALTER TABLE tasks DROP CONSTRAINT tasks_reference_id_unique;
+ALTER TABLE tasks ADD CONSTRAINT tasks_reference_id_unique UNIQUE (mitigation_id, reference_id);