test(e2e): isolate runs and identities without a new backend endpoint (WP-74)

The three specs shared one mutable backend and said so in their own
comments ("Restart the backend between CI runs"). WP-70 recorded the fix as
a dev-only seed endpoint; it isn't needed. The DB path already routes
through IConfiguration, so playwright.config's webServer hands the backend a
throwaway SQLite file per invocation — the same trick TestWebApplicationFactory
already uses, with zero backend change. And StubIdentityProvider already
honoured X-Subject; the only gap was that nothing sent it. That matters
because the backend has no IsDevelopment() gate anywhere, so a seed endpoint
would have had to invent the codebase's first environment gate.

subjectInterceptor mirrors the existing roleInterceptor and is wired into the
same isDevMode()-only list. Interceptors alone were not enough: the raw XHR
upload and the hand-written letter-preview fetch bypass Angular's chain (as
CLAUDE.md documents), so both now stamp X-Subject explicitly — without that,
every uploaded document still landed under DemoOwner.

reuseExistingServer stays on: flipping it would break local runs for anyone
already serving the docker stack. Each run gets a unique DB filename and
global-setup sweeps only prior runs' leftovers — deleting a fixed path
mid-run risks SQLite silently recreating an empty, unmigrated file under
fullyParallel.

Verified: e2e passes twice back-to-back with no backend restart, and
X-Subject was observed on a real request, not merely wired.

brief-v2.spec.ts keeps the shared identity for now — see the KNOWN GAP note;
a backend staleness bug makes /brief/preview return a sent letter with the
draft watermark for any non-DemoOwner BSN. actors.ts reserves the actor for
whoever fixes it.

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

apps/ssp/src/app/brief/infrastructure/letter-preview.adapter.ts

@@ -1,6 +1,7 @@
 import { Injectable } from '@angular/core';
 import { Result, ok, err } from '@shared/kernel/fp';
 import { currentRole } from '@shared/infrastructure/role';
+import { currentSubject } from '@shared/infrastructure/subject';
 import { problemDetail } from '@shared/infrastructure/api-error';
 import { environment } from '@shared/environments/environment';
 
@@ -12,15 +13,37 @@ export const PREVIEW_FAILED = $localize`:@@brief.preview.failed:De voorvertoning
  * `/brief/preview` returns `text/html`, not JSON, and is `.ExcludeFromDescription()`'d
  * to keep the NSwag-generated client JSON-only (same seam as uploads) — so this is a
  * hand-written fetch, not the `ApiClient`. That also means it bypasses `HttpClient`'s
- * `roleInterceptor`, so `X-Role` is set here explicitly.
+ * `roleInterceptor` AND `subjectInterceptor`, so both `X-Role` and `X-Subject` are set
+ * here explicitly (WP-74 — without `X-Subject` this always previewed
+ * `DocumentStore.DemoOwner`'s letter regardless of who was actually logged in).
+ *
+ * `cache: 'no-store'` (WP-74): the endpoint has no `Cache-Control`, only a CORS-driven
+ * `Vary: Origin`, and its content changes at the SAME URL as the letter moves
+ * draft → sent. Explicitly bypassing the HTTP cache is the correct default for any
+ * mutable resource served under one unversioned URL — independent of WP-74's
+ * identity work, and not a complete fix by itself: see the KNOWN GAP note below.
+ *
+ * KNOWN GAP (WP-74, not fixed here): under a non-`DocumentStore.DemoOwner` `X-Subject`,
+ * this repo's own e2e run against a real backend observed this endpoint's SENT
+ * response still carrying the draft watermark, even though (a) the outgoing request
+ * carried the correct `X-Subject`, and (b) `curl` against the same backend at the
+ * same moment correctly returned the frozen, unwatermarked archive. `cache: 'no-store'`
+ * did not change the outcome, so it is very unlikely a client-side caching artifact —
+ * it looks like a genuine backend-side staleness/race in `BriefStore`'s SQLite-backed
+ * read path, reproducible for MULTIPLE distinct owners and NOT reproducible for
+ * `DemoOwner`, which needs backend-side investigation (out of WP-74's file scope —
+ * see `e2e/brief-v2.spec.ts`'s header comment, which keeps that spec on the shared
+ * `zorgverlener` identity until this is root-caused).
  */
 @Injectable({ providedIn: 'root' })
 export class LetterPreviewAdapter {
   async preview(): Promise<Result<string, Blob>> {
     let res: Response;
     try {
+      const subject = currentSubject();
       res = await fetch(`${environment.apiBaseUrl}/api/v1/brief/preview`, {
-        headers: { 'X-Role': currentRole() },
+        cache: 'no-store',
+        headers: { 'X-Role': currentRole(), ...(subject ? { 'X-Subject': subject } : {}) },
       });
     } catch {
       return err(PREVIEW_FAILED);