✨ feat: add subscription billing system with admin management and user purchase flow

Implement a new subscription-based billing model alongside existing metered/per-request billing:

Backend:
- Add subscription plan models (SubscriptionPlan, SubscriptionPlanItem, UserSubscription, etc.)
- Implement CRUD APIs for subscription plan management (admin only)
- Add user subscription queries with support for multiple active/expired subscriptions
- Integrate payment gateways (Stripe, Creem, Epay) for subscription purchases
- Implement pre-consume and post-consume billing logic for subscription quota tracking
- Add billing preference settings (subscription_first, wallet_first, etc.)
- Enhance usage logs with subscription deduction details

Frontend - Admin:
- Add subscription management page with table view and drawer-based edit form
- Match UI/UX style with existing admin pages (redemption codes, users)
- Support enabling/disabling plans, configuring payment IDs, and model quotas
- Add user subscription binding modal in user management

Frontend - Wallet:
- Add subscription plans card with current subscription status display
- Show all subscriptions (active and expired) with remaining days/usage percentage
- Display purchasable plans with pricing cards following SaaS best practices
- Extract purchase modal to separate component matching payment confirm modal style
- Add skeleton loading states with active animation
- Implement billing preference selector in card header
- Handle payment gateway availability based on admin configuration

Frontend - Usage Logs:
- Display subscription deduction details in log entries
- Show step-by-step breakdown of subscription usage (pre-consumed, delta, final, remaining)
- Add subscription deduction tag for subscription-covered requests

web/src/components/table/users/modals/BindSubscriptionModal.jsx

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+/*
+Copyright (C) 2025 QuantumNous
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, either version 3 of the
+License, or (at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+For commercial licensing, please contact support@quantumnous.com
+*/
+
+import React, { useEffect, useMemo, useState } from 'react';
+import { Modal, Select, Space, Typography } from '@douyinfe/semi-ui';
+import { API, showError, showSuccess } from '../../../../helpers';
+
+const { Text } = Typography;
+
+const BindSubscriptionModal = ({ visible, onCancel, user, t, onSuccess }) => {
+  const [loading, setLoading] = useState(false);
+  const [plans, setPlans] = useState([]);
+  const [selectedPlanId, setSelectedPlanId] = useState(null);
+
+  const loadPlans = async () => {
+    setLoading(true);
+    try {
+      const res = await API.get('/api/subscription/admin/plans');
+      if (res.data?.success) {
+        setPlans(res.data.data || []);
+      } else {
+        showError(res.data?.message || t('加载失败'));
+      }
+    } catch (e) {
+      showError(t('请求失败'));
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  useEffect(() => {
+    if (visible) {
+      setSelectedPlanId(null);
+      loadPlans();
+    }
+  }, [visible]);
+
+  const planOptions = useMemo(() => {
+    return (plans || []).map((p) => ({
+      label: `${p?.plan?.title || ''} (${p?.plan?.currency || 'USD'} ${Number(p?.plan?.price_amount || 0)})`,
+      value: p?.plan?.id,
+    }));
+  }, [plans]);
+
+  const bind = async () => {
+    if (!user?.id) {
+      showError(t('用户信息缺失'));
+      return;
+    }
+    if (!selectedPlanId) {
+      showError(t('请选择订阅套餐'));
+      return;
+    }
+    setLoading(true);
+    try {
+      const res = await API.post('/api/subscription/admin/bind', {
+        user_id: user.id,
+        plan_id: selectedPlanId,
+      });
+      if (res.data?.success) {
+        showSuccess(t('绑定成功'));
+        onSuccess?.();
+        onCancel?.();
+      } else {
+        showError(res.data?.message || t('绑定失败'));
+      }
+    } catch (e) {
+      showError(t('请求失败'));
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  return (
+    <Modal
+      title={t('绑定订阅套餐')}
+      visible={visible}
+      onCancel={onCancel}
+      onOk={bind}
+      confirmLoading={loading}
+      maskClosable={false}
+      centered
+    >
+      <Space vertical style={{ width: '100%' }} spacing='medium'>
+        <div className='text-sm'>
+          <Text strong>{t('用户')}：</Text>
+          <Text>{user?.username}</Text>
+          <Text type='tertiary'> (ID: {user?.id})</Text>
+        </div>
+        <Select
+          placeholder={t('选择订阅套餐')}
+          optionList={planOptions}
+          value={selectedPlanId}
+          onChange={setSelectedPlanId}
+          loading={loading}
+          filter
+          style={{ width: '100%' }}
+        />
+        <div className='text-xs text-gray-500'>
+          {t('绑定后会立即生成用户订阅（无需支付），有效期按套餐配置计算。')}
+        </div>
+      </Space>
+    </Modal>
+  );
+};
+
+export default BindSubscriptionModal;
+