feat: redeem items support optional duration (permanent/timed)

- Add duration_minutes column to redeem_items (0 = permanent)
- Backend: update RedeemItem/RedeemItemWrite structs, all CRUD SQL
- Backend: EnsureColumn migration for existing databases
- Frontend: add duration selector dropdown (永久/15min/30min/1h/2h/3h/custom)
- Frontend: show ♾️ Permanent or ⏱️ duration in item list

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/include/csp/services/redeem_service.h

@@ -16,6 +16,7 @@ struct RedeemItem {
   std::string unit_label;
   int holiday_cost = 0;
   int studyday_cost = 0;
+  int duration_minutes = 0;  // 0 = permanent
   bool is_active = true;
   bool is_global = true;
   int64_t created_by = 0;
@@ -29,6 +30,7 @@ struct RedeemItemWrite {
   std::string unit_label = "小时";
   int holiday_cost = 5;
   int studyday_cost = 25;
+  int duration_minutes = 0;  // 0 = permanent
   bool is_active = true;
   bool is_global = true;
 };