fix(structures): align Structure.delegation_code FK action with Prisma default (#229)

## Summary

Single-line fix on the `add_org_hierarchy` migration shipped in [#228](#228) (ADR-0027 PR 1). The FK action on `Structure.delegation_code` was `ON DELETE RESTRICT` (Prisma's default for **required** relations); the field is **optional** (`Structure.delegation Delegation?`), so Prisma's 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 (the workspace dev DB). No production deployment of #228 yet, so the fix is **edit-in-place** rather than a corrective sibling migration — keeps the repo's migration history clean.

## What lands

| File | Change |
| --- | --- |
| `apps/portal-bff/prisma/migrations/20260526143000_add_org_hierarchy/migration.sql` | `ON DELETE RESTRICT` → `ON DELETE SET NULL` on the `structures_delegation_code_fkey` FK. Inline comment explains the rule (nullable Prisma relation → SET NULL is the matching default; not matching it generates drift on every `migrate dev`). |

That's it. One line of SQL, one comment block. No schema.prisma change, no test change, no other file touched.

## Why edit-in-place vs new corrective migration

Edit-in-place is safe here because:

- The broken migration only exists in **dev local DBs** (Julien's WSL postgres). No staging, no preview env, no prod has applied it.
- Every dev who pulls this PR's fix wipes their local DB (`./infra/local/dev.sh down -v`) and reapplies — clean migration history, no "modified after applied" warning.
- The repo's migration list stays minimal — adding a corrective migration would carry the wart forever in `prisma/migrations/`.

Once a non-dev environment has applied a migration, the rule reverses: corrective migration mandatory, never edit in place. ADR-0015's "trunk-based + squash-merge" and the absence of a deployed environment at this stage gives us this one-time window.

## Recovery procedure for anyone who applied #228

```bash
# From the workspace root
./infra/local/dev.sh down -v   # wipe the local postgres volume
./infra/local/dev.sh up

# Pull this fix
git pull   # or git switch fix/adr-0027-pr1-fk-action-on-delete depending on local state

# Reapply migrations — no prompt this time
cd apps/portal-bff && pnpm exec prisma migrate dev
# Should report: "Applied migration `20260526143000_add_org_hierarchy`" and exit cleanly.
```

If anyone has a stray `drift_inspection/` folder under `prisma/migrations/` (artifact of the `--create-only` debugging step), delete it before reapplying — it's not in the repo, it was just diagnostic output.

## Test plan

- [x] `node scripts/check-catalogue-drift.mjs` — clean (unchanged by this fix).
- [x] `node --test scripts/check-catalogue-drift.spec.mjs` — 22 tests passing (unchanged).
- [x] `pnpm exec prettier --check` clean (SQL file unaffected by prettier but verified).
- [ ] **Locally on WSL** — wipe DB → pull fix → `prisma migrate dev` exits clean, no drift prompt.
- [ ] **Review focus** — the comment block in the migration explaining the rule (future-proof against the same mistake when ADR-0026 PR 1 adds optional relations).

## Notes for the reviewer

- **Why a comment block on the FK line, not on every nullable FK in future migrations?** This was caught the first time we hit it; a short note at the offending site is cheaper than amending `CLAUDE.md` with a "remember to match Prisma default actions" rule. If we hit the same mistake on ADR-0026 PR 1's `Person`/`User`/`UserScope` migration, we'll consider promoting it. For now: one inline comment at the place where the rule is non-obvious.
- **The drift gate doesn't catch this kind of mistake.** Catalogue drift gate scans string literals against TypeScript catalogues — it doesn't look at SQL referential actions. Postgres CHECK constraints (introduced for `Structure.kind` in this same migration) are not the same surface as FK referential actions. Catching this required actually running `prisma migrate dev`. A possible future improvement: a CI gate that runs `prisma migrate diff --from-migrations --to-schema-datamodel --script --shadow-database-url …` and fails if the diff is non-empty (zero-drift gate). Worth an ADR amendment if it becomes a recurring class of bug.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #229

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").