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>

frontend/src/app/admin-redeem/page.tsx

@@ -14,6 +14,7 @@ type RedeemItem = {
   unit_label: string;
   holiday_cost: number;
   studyday_cost: number;
+  duration_minutes: number;
   is_active: boolean;
   is_global: boolean;
   created_at: number;
@@ -40,6 +41,7 @@ type ItemForm = {
   unit_label: string;
   holiday_cost: number;
   studyday_cost: number;
+  duration_minutes: number;
   is_active: boolean;
   is_global: boolean;
 };
@@ -50,6 +52,7 @@ const DEFAULT_FORM: ItemForm = {
   unit_label: "hour",
   holiday_cost: 5,
   studyday_cost: 25,
+  duration_minutes: 0,
   is_active: true,
   is_global: true,
 };
@@ -148,6 +151,7 @@ export default function AdminRedeemPage() {
       unit_label: item.unit_label,
       holiday_cost: item.holiday_cost,
       studyday_cost: item.studyday_cost,
+      duration_minutes: item.duration_minutes,
       is_active: item.is_active,
       is_global: item.is_global,
     });
@@ -217,6 +221,34 @@ export default function AdminRedeemPage() {
               setForm((prev) => ({ ...prev, studyday_cost: Math.max(0, Number(e.target.value) || 0) }))
             }
           />
+          <div className="flex items-center gap-2">
+            <select
+              className="rounded border px-3 py-2 text-sm flex-1"
+              value={form.duration_minutes === 0 ? "0" : String(form.duration_minutes)}
+              onChange={(e) => setForm((prev) => ({ ...prev, duration_minutes: Number(e.target.value) }))}
+            >
+              <option value="0">{tx("永久有效 ♾️", "Permanent ♾️")}</option>
+              <option value="15">15 {tx("分钟", "min")}</option>
+              <option value="30">30 {tx("分钟", "min")}</option>
+              <option value="60">1 {tx("小时", "hour")}</option>
+              <option value="90">1.5 {tx("小时", "hours")}</option>
+              <option value="120">2 {tx("小时", "hours")}</option>
+              <option value="180">3 {tx("小时", "hours")}</option>
+              <option value="-1">{tx("自定义", "Custom")}</option>
+            </select>
+            {form.duration_minutes === -1 && (
+              <input
+                className="rounded border px-3 py-2 text-sm w-24"
+                type="number"
+                min={1}
+                placeholder={tx("分钟", "min")}
+                onChange={(e) => {
+                  const v = Math.max(1, Number(e.target.value) || 1);
+                  setForm((prev) => ({ ...prev, duration_minutes: v }));
+                }}
+              />
+            )}
+          </div>
           <textarea
             className="rounded border px-3 py-2 text-sm md:col-span-2"
             placeholder={tx("描述", "Description")}
@@ -291,6 +323,12 @@ export default function AdminRedeemPage() {
               <p>
                 #{item.id} · {item.name} · {tx("假期", "Holiday")} {item.holiday_cost}/{item.unit_label} · {tx("学习日", "Study Day")} {item.studyday_cost}/
                 {item.unit_label}
+                {" · "}
+                {item.duration_minutes > 0
+                  ? (item.duration_minutes >= 60
+                    ? `⏱️ ${item.duration_minutes / 60}${tx("小时", "h")}`
+                    : `⏱️ ${item.duration_minutes}${tx("分钟", "min")}`)
+                  : `♾️ ${tx("永久", "Permanent")}`}
               </p>
               <p className="text-xs text-zinc-600">
                 {tx("状态：", "Status: ")}