Restrict deleting a coordinator who still has assigned volunteers

The old single-column FK blocked this; keep that behavior on the join table
instead of cascading the assignments away.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Juwang110

apps/backend/src/coordinators/coordinators.entity.ts

@@ -36,6 +36,8 @@ export class FosterCoordinator {
   @ManyToMany(
     () => FosterVolunteer,
     (volunteer) => volunteer.assignedCoordinators,
+    // Matches the migration: a coordinator with assigned volunteers can't be deleted.
+    { onDelete: 'RESTRICT' },
   )
   assignedVolunteers!: FosterVolunteer[];
 }

apps/backend/src/migrations/1791100000000-ReplaceAssignedCoordinatorWithJoinTable.ts

@@ -17,12 +17,13 @@ export class ReplaceAssignedCoordinatorWithJoinTable1791100000000
                 CONSTRAINT "fk_assignment_coordinator"
                     FOREIGN KEY ("coordinator_id")
                     REFERENCES "foster_coordinators" ("coordinator_id")
-                    ON DELETE CASCADE
+                    ON DELETE RESTRICT
             )
         `);
 
-    // The composite PK leads with volunteer_id, so lookups and cascades by
-    // coordinator need their own index.
+    // Deleting a coordinator who still has volunteers is blocked, as it was with
+    // the old single-column FK. The composite PK leads with volunteer_id, so
+    // lookups and that delete check by coordinator need their own index.
     await queryRunner.query(`
             CREATE INDEX "idx_volunteer_coordinator_assignments_coordinator_id"
                 ON "volunteer_coordinator_assignments" ("coordinator_id")