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.

backend/app/api/group_routes.py

@@ -338,6 +338,53 @@ def update_persona(gid: str, pid: str):
     return jsonify({"persona": strip_secret_fields(full)})
 
 
+@groups_bp.post("/<gid>/personas/<pid>/variant")
+@require_auth
+def create_persona_variant(gid: str, pid: str):
+    """Create a NEW persona cloned from an existing one (fresh identity, same core traits).
+
+    Lets a trainee practice the SAME selling challenge repeatedly even though each persona
+    can only be chatted once — the variant is a different person with the same pain points /
+    personality / temperament, so the training repeats but never as an identical copy.
+    Anyone who has access to the group can create a variant (admin or trainee).
+    """
+    s = _stores()
+    group = _get_owned_group(s, gid)
+    if group.get("status") != "ready":
+        raise ApiError("group not ready", 403)
+    src = next((p for p in group.get("personas", []) if p.get("id") == pid), None)
+    if src is None:
+        raise ApiError("persona not found", 404)
+
+    lang = (group.get("input") or {}).get("language", "th")
+    try:
+        from ..services.persona_generator import PersonaGenerator
+        variant = PersonaGenerator(s["llm"]).generate_variant(
+            source=src,
+            sales_kit=group.get("sales_kit") or {},
+            language=lang,
+        )
+    except Exception as exc:
+        raise ApiError(f"variant failed: {exc}", 500)
+
+    # Assign a unique id and append to the group (keeps existing personas/sessions intact).
+    import uuid as _uuid
+    variant["id"] = f"persona-{_uuid.uuid4().hex[:10]}"
+    variant["source_persona_id"] = pid
+    personas = group.get("personas", []) + [variant]
+    s["groups"].update(gid, personas=personas)
+    full = ensure_persona_shape(variant)
+
+    actor = current_user()
+    if actor.get("role") == "super_admin":
+        persona_out = full
+    elif actor.get("role") == "user":
+        persona_out = revealable_view(full)
+    else:
+        persona_out = strip_secret_fields(full)
+    return jsonify({"persona": persona_out}), 201
+
+
 @groups_bp.delete("/<gid>")
 @require_auth
 @require_roles("admin")