feat(call2): short/long desc structure in seed (not brand voice)

Call 2 seed now requires benefit-first short + four-part long HTML, returned
as JSON. POST parses and persists both fields; Create-desc gate no longer
requires short (Call 2 writes it). Brand voice/geo stay Settings-injected.

frontend/src/api/slabs.ts

@@ -364,6 +364,8 @@ export interface Call1Result {
 export interface Call2Result {
   ok: boolean;
   description: string;
+  /** Present when Call 2 returned a parsed short_description. */
+  short_description?: string;
 }
 
 /** Call 1 OUT (overnight-whole-app): always fails with 410 call1_disabled. Prefer manual taxonomy. */