feat(personas): 'create persona from this persona' (variant) — practice the same challenge, new identity

Each persona is one-shot (chat once = win/lose locked). To keep training repeatable:
- New endpoint POST /api/groups/<gid>/personas/<pid>/variant creates a NEW persona that is
  a fresh incarnation of the source: LOCKS pain points, objections, negotiation levers,
  tolerance, special/recontact, goal, budget, difficulty, tier, product_context — but VARYS
  name/profession/age/location/background/personality/opener so it isn't an identical copy.
- Added to the same group as a distinct persona (fresh not_tried, so chat-able again).
- UI: on the Personas page, a finished (won/lost) persona gets a
  'สร้างบุคคลต้นแบบจากต้นแบบนี้' button; reload shows the variant.
All 10 backend suites pass. Rebuilt dist.

frontend/dist/assets/layout-dashboard-qMD4csbw.js

@@ -0,0 +1,6 @@
+import{c as t}from"./index-C--0e2U-.js";/**
+ * @license lucide-vue-next v1.0.0 - ISC
+ *
+ * This source code is licensed under the ISC license.
+ * See the LICENSE file in the root directory of this source tree.
+ */const h=t("layout-dashboard",[["rect",{width:"7",height:"9",x:"3",y:"3",rx:"1",key:"10lvy0"}],["rect",{width:"7",height:"5",x:"14",y:"3",rx:"1",key:"16une8"}],["rect",{width:"7",height:"9",x:"14",y:"12",rx:"1",key:"1hutg5"}],["rect",{width:"7",height:"5",x:"3",y:"16",rx:"1",key:"ldoo1y"}]]);export{h as L};