fix(brief): make GET /brief a pure query, 404 when absent (RB-23)

GET /brief allocated a row on first call (BriefStore.GetOrCreate) — the
one endpoint in the backend where a read performed a persisted write.
The FE retries GETs automatically, so a transient failure could enter
the create path more than once; a lock prevented a duplicate row, but
the safety depended on the lock, not on the endpoint being a query.

Split GetOrCreate into Get (a pure query) and the already-existing
ResetAndCreate (POST /brief/reset owns creation). GET /brief now 404s
when the owner has no brief yet. GET /brief/preview used GetOrCreate
too, so it gets the same Get + 404 treatment, forced by the split.

RB-22 already made BriefStore.load() on the FE tolerate a 404 by
calling reset() once; this ticket is what makes that branch live.

Updated the brief/preview/org-template backend tests that assumed
GET seeded a brief on first call to create one explicitly first, and
added a test that GET 404s and writes no row without the fix (verified
red beforehand). Regenerated the API client (npm run gen:api).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

libs/shared/docs/behaviour-spec.mdx

@@ -21,7 +21,7 @@ tested where._
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
 **is** the suite, reshaped for a business reader. 467 frontend behaviours across
-9 contexts; 237 backend behaviours across 41 test
+9 contexts; 238 backend behaviours across 41 test
 classes.
 
 ## Frontend (by context)
@@ -999,7 +999,8 @@ classes.
 
 ### BriefEndpointTests
 
-- Get creates a draft with expected sections locked and empty
+- Get returns 404 and writes no row when no brief exists for the owner
+- SeedBrief creates a draft with expected sections locked and empty
 - Get offers only global and arts scoped besluit tagged passages
 - Get joins the case context with the BIG nummer masked
 - Reveal returns the unmasked BIG nummer for the drafter with step up

libs/shared/src/infrastructure/api-client.ts

@@ -1350,6 +1350,10 @@ export class ApiClient {
             result200 = _responseText === "" ? null : JSON.parse(_responseText, this.jsonParseReviver) as BriefViewDto;
             return result200;
             });
+        } else if (status === 404) {
+            return response.text().then((_responseText) => {
+            return throwException("Not Found", status, _responseText, _headers);
+            });
         } else if (status !== 200 && status !== 204) {
             return response.text().then((_responseText) => {
             return throwException("An unexpected server error occurred.", status, _responseText, _headers);