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/behandelportal/src/app/app.config.ts

@@ -14,6 +14,7 @@ import localeEn from '@angular/common/locales/en';
 import { routes } from './app.routes';
 import { scenarioInterceptor } from '@shared/infrastructure/scenario.interceptor';
 import { roleInterceptor } from '@shared/infrastructure/role.interceptor';
+import { subjectInterceptor } from '@shared/infrastructure/subject.interceptor';
 import { medewerkerInterceptor } from '@auth/infrastructure/medewerker.interceptor';
 import { provideApiClient } from '@shared/infrastructure/api-client.provider';
 import { SESSION_PORT } from '@shared/application/session.port';
@@ -55,7 +56,9 @@ export const appConfig: ApplicationConfig = {
     // a query param could otherwise force errors on the live app.
     provideHttpClient(
       withInterceptors(
-        isDevMode() ? [scenarioInterceptor, roleInterceptor, medewerkerInterceptor] : [],
+        isDevMode()
+          ? [scenarioInterceptor, roleInterceptor, subjectInterceptor, medewerkerInterceptor]
+          : [],
       ),
     ),
     provideApiClient(),