feat: add platform type filter to subscription management page

Add a platform filter dropdown to the admin subscriptions view, allowing
filtering subscriptions by platform (Anthropic, OpenAI, Gemini, etc.)
through the group association.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/admin/subscriptions.ts

@@ -27,6 +27,7 @@ export async function list(
     status?: 'active' | 'expired' | 'revoked'
     user_id?: number
     group_id?: number
+    platform?: string
     sort_by?: string
     sort_order?: 'asc' | 'desc'
   },

frontend/src/i18n/locales/en.ts

@@ -1694,6 +1694,7 @@ export default {
       revokeSubscription: 'Revoke Subscription',
       allStatus: 'All Status',
       allGroups: 'All Groups',
+      allPlatforms: 'All Platforms',
       daily: 'Daily',
       weekly: 'Weekly',
       monthly: 'Monthly',

frontend/src/i18n/locales/zh.ts

@@ -1774,6 +1774,7 @@ export default {
       revokeSubscription: '撤销订阅',
       allStatus: '全部状态',
       allGroups: '全部分组',
+      allPlatforms: '全部平台',
       daily: '每日',
       weekly: '每周',
       monthly: '每月',

frontend/src/views/admin/SubscriptionsView.vue

@@ -81,6 +81,14 @@
                 @change="applyFilters"
               />
             </div>
+            <div class="w-full sm:w-40">
+              <Select
+                v-model="filters.platform"
+                :options="platformFilterOptions"
+                :placeholder="t('admin.subscriptions.allPlatforms')"
+                @change="applyFilters"
+              />
+            </div>
           </div>
 
           <!-- Right: Actions -->
@@ -813,6 +821,7 @@ let userSearchTimeout: ReturnType<typeof setTimeout> | null = null
 const filters = reactive({
   status: 'active',
   group_id: '',
+  platform: '',
   user_id: null as number | null
 })
 
@@ -855,6 +864,15 @@ const groupOptions = computed(() => [
   ...groups.value.map((g) => ({ value: g.id.toString(), label: g.name }))
 ])
 
+const platformFilterOptions = computed(() => [
+  { value: '', label: t('admin.subscriptions.allPlatforms') },
+  { value: 'anthropic', label: 'Anthropic' },
+  { value: 'openai', label: 'OpenAI' },
+  { value: 'gemini', label: 'Gemini' },
+  { value: 'antigravity', label: 'Antigravity' },
+  { value: 'sora', label: 'Sora' }
+])
+
 // Group options for assign (only subscription type groups)
 const subscriptionGroupOptions = computed(() =>
   groups.value
@@ -890,6 +908,7 @@ const loadSubscriptions = async () => {
       {
         status: (filters.status as any) || undefined,
         group_id: filters.group_id ? parseInt(filters.group_id) : undefined,
+        platform: filters.platform || undefined,
         user_id: filters.user_id || undefined,
         sort_by: sortState.sort_by,
         sort_order: sortState.sort_order