fix(structures): align Structure.delegation_code FK action with Prisma default

ON DELETE RESTRICT was Prisma's default for required relations;
Structure.delegation is optional (`Delegation?`), so the matching
default is ON DELETE SET NULL. Mismatch caused `prisma migrate dev`
to detect drift and prompt for a corrective migration on every run.

Caught locally on first VM-side validation. No production deployment
of #228 yet, so editing the migration in place rather than shipping a
corrective sibling - keeps the repo's migration history minimal.

Inline comment at the offending line explains the rule (nullable
Prisma relation -> SET NULL) to head off the same mistake when
ADR-0026 PR 1 adds optional FKs (Person.user, User.scopes).

apps/portal-bff/prisma/migrations/20260526143000_add_org_hierarchy/migration.sql

@@ -83,9 +83,13 @@ ALTER TABLE "delegations" ADD CONSTRAINT "delegations_region_code_fkey"
     ON DELETE RESTRICT ON UPDATE CASCADE;
 
 -- AddForeignKey
+-- ON DELETE SET NULL because delegationCode is optional in the Prisma
+-- schema (Structure.delegation Delegation?). Matches Prisma's default
+-- for nullable relations; the drift gate would otherwise flag this
+-- on every `prisma migrate dev`.
 ALTER TABLE "structures" ADD CONSTRAINT "structures_delegation_code_fkey"
     FOREIGN KEY ("delegation_code") REFERENCES "delegations"("code")
-    ON DELETE RESTRICT ON UPDATE CASCADE;
+    ON DELETE SET NULL ON UPDATE CASCADE;
 
 -- ------------------------------------------------------------------
 -- Test-tenant reference seed (per ADR-0027 §"Seeding posture").