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/swagger.json

@@ -863,6 +863,26 @@
           }
         }
       }
+    },
+    "/api/v1/brief/reset": {
+      "post": {
+        "tags": [
+          "BigRegister.Api, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"
+        ],
+        "operationId": "briefReset",
+        "responses": {
+          "200": {
+            "description": "OK",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/BriefViewDto"
+                }
+              }
+            }
+          }
+        }
+      }
     }
   },
   "components": {
@@ -1389,6 +1409,9 @@
               "$ref": "#/components/schemas/LetterBlockDto"
             },
             "nullable": true
+          },
+          "locked": {
+            "type": "boolean"
           }
         },
         "additionalProperties": false
@@ -1455,6 +1478,10 @@
               "$ref": "#/components/schemas/RichTextNodeDto"
             },
             "nullable": true
+          },
+          "list": {
+            "type": "string",
+            "nullable": true
           }
         },
         "additionalProperties": false