feat(brief): locked sections, list formatting, auto/manual placeholder chips

- brief.machine: reducer refuses edits to locked (predefined) sections as
  defense-in-depth; LetterSection gains a `locked` flag
- rich-text: paragraphs gain optional `list` kind; editor gets bullet/numbered
  list buttons, keyboard shortcuts, and backspace-deletes-adjacent-chip
- placeholder chips distinguish auto-resolvable (grey) vs manual (yellow), in
  both the editor and the read-only preview
- fix: preview chip now renders matching {…} braces (was a one-sided ⌗ glyph),
  aligned with the editor's chip styling

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -304,6 +304,15 @@ api.MapPost("/brief/send", () =>
 .Produces<BriefDto>()
 .ProducesProblem(StatusCodes.Status409Conflict);
 
+api.MapPost("/brief/reset", () =>
+{
+    // Demo "start over": recreate a fresh draft. No guards — showcase affordance only.
+    var e = BriefStore.ResetAndCreate(DocumentStore.DemoOwner);
+    return new BriefViewDto(e.ToDto(), BriefSeed.PassagesFor(e.Beroep));
+})
+.WithName("briefReset")
+.Produces<BriefViewDto>();
+
 app.Run();
 
 static bool IsAdmin(HttpContext ctx) => ctx.Request.Headers["X-Admin"] == "true";