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/src/db/sqlite_db.cc

@@ -524,6 +524,7 @@ CREATE TABLE IF NOT EXISTS redeem_items (
   unit_label TEXT NOT NULL DEFAULT "小时",
   holiday_cost INTEGER NOT NULL DEFAULT 5,
   studyday_cost INTEGER NOT NULL DEFAULT 25,
+  duration_minutes INTEGER NOT NULL DEFAULT 0,
   is_active INTEGER NOT NULL DEFAULT 1,
   is_global INTEGER NOT NULL DEFAULT 1,
   created_by INTEGER NOT NULL DEFAULT 0,
@@ -594,6 +595,8 @@ CREATE TABLE IF NOT EXISTS daily_task_logs (
   EnsureColumn(db, "problem_solutions", "complexity",
                "complexity TEXT NOT NULL DEFAULT ''");
   EnsureColumn(db, "problem_solutions", "tags_json", "tags_json TEXT NOT NULL DEFAULT '[]'");
+  EnsureColumn(db, "redeem_items", "duration_minutes",
+               "duration_minutes INTEGER NOT NULL DEFAULT 0");
 
   // Build indexes after compatibility ALTERs so old schemas won't fail on
   // missing columns (e.g. legacy submissions table without contest_id).