feat(form-builder): denormalize organisation_id and event_id on form_submissions per addendum Q2

Adds direct tenant + event columns to form_submissions so rapportage-hot
aggregate queries (dashboards, CSV-exports, counts over thousands of rows
per org or per event) skip the form_schemas join. This is the single
denormalization exception per addendum Q2; every other form-builder child
table continues to resolve tenancy via FK-chain through its parent
(implemented in Commit 3).

Schema:
- form_submissions.organisation_id  ULID FK → organisations, cascade delete, NOT NULL
- form_submissions.event_id          ULID FK → events, null on delete, nullable
- Indexes: (organisation_id, status), (event_id, status)

Observer: App\Observers\FormBuilder\FormSubmissionObserver::creating
resolves both columns when the caller has not set them.
  - organisation_id <- form_schema.organisation_id (always present —
    form_schemas carries OrganisationScope's column directly)
  - event_id <- schema.owner_id when owner_type === 'event'; else the
    active route's {event} parameter; else null (user_profile /
    signature_contract purposes)
The observer docblock spells out both resolution paths and is covered
by the observer test below.

Model: FormSubmission gains organisation_id + event_id in $fillable, a
belongsTo organisation() and belongsTo event() relation.

Factory: FormSubmissionFactory gains forOrganisation($org) and
forEvent($event) states for tests that need to override the observer's
automatic resolution (e.g. cross-org leakage scenarios in Commit 3).
Normal factory usage does not need the states — the observer populates
both fields on save.

Docs:
- SCHEMA.md §3.5.12 form_submissions table — organisation_id and event_id
  inserted between form_schema_id and subject_type; indexes added;
  addendum Q2 rationale paragraph at the bottom explaining why this is
  the only denormalized form-builder child.
- ARCH-FORM-BUILDER.md §4.3 — mirror changes + rationale inline on the
  columns and in the indexes list.

Tests: tests/Feature/FormBuilder/FormSubmissionObserverTest.php — 7 tests
covering organisation resolution from schema, event resolution from
event-owned schema, null event_id for non-event-owned schemas without
route context, route-based event resolution, organisation_id populated
on every create path (factory / new() / Model::create), index presence,
and belongsTo relations. 13 new assertions. Full suite: 984 passed
(2675 assertions).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dev-docs/ARCH-FORM-BUILDER.md

@@ -666,6 +666,8 @@ submissions of the same schema. Enforcement:
 |---|---|---|
 | `id` | ULID | PK |
 | `form_schema_id` | ULID FK | → form_schemas |
+| `organisation_id` | ULID FK | → organisations, cascade delete. **Denormalized per ARCH-CONSOLIDATION-ADDENDUM-2026-04-24 Q2** — the single rapportage-hot exception. Populated by `FormSubmissionObserver::creating` from the schema parent. |
+| `event_id` | ULID FK nullable | → events, null on delete. Denormalized per addendum Q2. Observer resolves from `form_schemas.owner_id` when `owner_type = event`; else from the active route's `{event}` parameter. Null for purposes without an event context (`user_profile`, `signature_contract`). |
 | `subject_type` | string nullable | polymorph |
 | `subject_id` | ULID nullable | polymorph target |
 | `submitted_by_user_id` | ULID FK nullable | Authenticated submitter |
@@ -694,7 +696,7 @@ submissions of the same schema. Enforcement:
 
 **Relations:** belongsTo form_schema, hasMany form_values, hasMany form_submission_section_statuses, hasMany form_submission_delegations, belongsTo submittedBy / reviewedBy (User), morphsTo subject
 
-**Indexes:** `(form_schema_id, status)`, `(subject_type, subject_id)`, `(submitted_by_user_id)`, `(form_schema_id, review_status)` partial where not null, `FULLTEXT(search_index)` (MySQL)
+**Indexes:** `(form_schema_id, status)`, `(organisation_id, status)` (addendum Q2 — dashboards + CSV exports aggregate by tenant + status), `(event_id, status)` (addendum Q2 — event-scoped reporting), `(subject_type, subject_id)`, `(submitted_by_user_id)`, `(form_schema_id, review_status)` partial where not null, `FULLTEXT(search_index)` (MySQL)
 
 **Unique:** `(form_schema_id, idempotency_key)` partial where not null