fix(ci): unbreak backend format, storybook-a11y, and e2e jobs

- backend: dotnet format the WP-51 migration (2-space indent, no BOM)
  to match .editorconfig — dotnet format --verify-no-changes was failing.
- storybook: stub FeatureFlagStore (WP-47) in shell/site-header stories
  alongside AccessStore, fixing NG0201 no-provider errors; bump the
  storybook-a11y container's memory cap 4g→6g (build-storybook +
  compodoc measured ~5.8GB peak RSS, leaving too little headroom).
- backend: fix a startup-breaking bug in the new (WP-52) POST
  /zgw/notificaties handler — it took ZgwOptions as a minimal-API
  parameter, which isn't registered in DI, so ASP.NET's endpoint-table
  build threw on every request once the route was registered (incl.
  /swagger, which is why Playwright's webServer health check timed
  out). Close over the existing `zgw` local instead.
- e2e: brief-v2.spec.ts's "Voorbeeld" button locator was ambiguous
  once a second "Voorbeeld met testwaarden" button existed (Playwright
  name matching is substring-based) — added `exact: true`. Also fixed
  the sent-letter preview flow to match app-letter-composer's actual
  behavior (single click → fetch, no in-page dialog, unlike
  app-behandel-scherm's), and fixed a watermark assertion that checked
  for the always-present `.preview-watermark` CSS class name instead
  of the conditionally-rendered "VOORBEELD" marker text.

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

backend/src/BigRegister.Api/Data/Migrations/20260729071227_ZaakAndDrcUrls.cs

@@ -1,38 +1,38 @@
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
 
 namespace BigRegister.Api.Data.Migrations
 {
+  /// <inheritdoc />
+  public partial class ZaakAndDrcUrls : Migration
+  {
     /// <inheritdoc />
-    public partial class ZaakAndDrcUrls : Migration
+    protected override void Up(MigrationBuilder migrationBuilder)
     {
-        /// <inheritdoc />
-        protected override void Up(MigrationBuilder migrationBuilder)
-        {
-            migrationBuilder.AddColumn<string>(
-                name: "DrcUrl",
-                table: "Documents",
-                type: "TEXT",
-                nullable: true);
+      migrationBuilder.AddColumn<string>(
+          name: "DrcUrl",
+          table: "Documents",
+          type: "TEXT",
+          nullable: true);
 
-            migrationBuilder.AddColumn<string>(
-                name: "ZaakUrl",
-                table: "Applications",
-                type: "TEXT",
-                nullable: true);
-        }
-
-        /// <inheritdoc />
-        protected override void Down(MigrationBuilder migrationBuilder)
-        {
-            migrationBuilder.DropColumn(
-                name: "DrcUrl",
-                table: "Documents");
-
-            migrationBuilder.DropColumn(
-                name: "ZaakUrl",
-                table: "Applications");
-        }
+      migrationBuilder.AddColumn<string>(
+          name: "ZaakUrl",
+          table: "Applications",
+          type: "TEXT",
+          nullable: true);
     }
+
+    /// <inheritdoc />
+    protected override void Down(MigrationBuilder migrationBuilder)
+    {
+      migrationBuilder.DropColumn(
+          name: "DrcUrl",
+          table: "Documents");
+
+      migrationBuilder.DropColumn(
+          name: "ZaakUrl",
+          table: "Applications");
+    }
+  }
 }