feat(antigravity): add 403 forbidden status detection, classification and display

Backend:
- Detect and classify 403 responses into three types:
  validation (account needs Google verification),
  violation (terms of service / banned),
  forbidden (generic 403)
- Extract verification/appeal URLs from 403 response body
  (structured JSON parsing with regex fallback)
- Add needs_verify, is_banned, needs_reauth, error_code fields
  to UsageInfo (omitempty for zero impact on other platforms)
- Handle 403 in request path: classify and permanently set account error
- Save validation_url in error_message for degraded path recovery
- Enrich usage with account error on both success and degraded paths
- Add singleflight dedup for usage requests with independent context
- Differentiate cache TTL: success/403 → 3min, errors → 1min
- Return degraded UsageInfo instead of HTTP 500 on quota fetch errors

Frontend:
- Display forbidden status badges with color coding (red for banned,
  amber for needs verification, gray for generic)
- Show clickable verification/appeal URL links
- Display needs_reauth and degraded error states in usage cell
- Add Antigravity tier label badge next to platform type

Tests:
- Comprehensive unit tests for classifyForbiddenType (7 cases)
- Unit tests for extractValidationURL (8 cases including unicode escapes)
- Integration test for FetchQuota forbidden path

frontend/src/views/admin/AccountsView.vue

@@ -171,7 +171,15 @@
             <span v-else class="text-sm text-gray-400 dark:text-dark-500">-</span>
           </template>
           <template #cell-platform_type="{ row }">
-            <PlatformTypeBadge :platform="row.platform" :type="row.type" :plan-type="row.credentials?.plan_type" :privacy-mode="row.extra?.privacy_mode" />
+            <div class="flex flex-wrap items-center gap-1">
+              <PlatformTypeBadge :platform="row.platform" :type="row.type" :plan-type="row.credentials?.plan_type" :privacy-mode="row.extra?.privacy_mode" />
+              <span
+                v-if="getAntigravityTierLabel(row)"
+                :class="['inline-block rounded px-1.5 py-0.5 text-[10px] font-medium', getAntigravityTierClass(row)]"
+              >
+                {{ getAntigravityTierLabel(row) }}
+              </span>
+            </div>
           </template>
           <template #cell-capacity="{ row }">
             <AccountCapacityCell :account="row" />
@@ -794,6 +802,40 @@ const { pause: pauseAutoRefresh, resume: resumeAutoRefresh } = useIntervalFn(
   { immediate: false }
 )
 
+// Antigravity 订阅等级辅助函数
+function getAntigravityTierFromRow(row: any): string | null {
+  if (row.platform !== 'antigravity') return null
+  const extra = row.extra as Record<string, unknown> | undefined
+  if (!extra) return null
+  const lca = extra.load_code_assist as Record<string, unknown> | undefined
+  if (!lca) return null
+  const paid = lca.paidTier as Record<string, unknown> | undefined
+  if (paid && typeof paid.id === 'string') return paid.id
+  const current = lca.currentTier as Record<string, unknown> | undefined
+  if (current && typeof current.id === 'string') return current.id
+  return null
+}
+
+function getAntigravityTierLabel(row: any): string | null {
+  const tier = getAntigravityTierFromRow(row)
+  switch (tier) {
+    case 'free-tier': return t('admin.accounts.tier.free')
+    case 'g1-pro-tier': return t('admin.accounts.tier.pro')
+    case 'g1-ultra-tier': return t('admin.accounts.tier.ultra')
+    default: return null
+  }
+}
+
+function getAntigravityTierClass(row: any): string {
+  const tier = getAntigravityTierFromRow(row)
+  switch (tier) {
+    case 'free-tier': return 'bg-gray-100 text-gray-600 dark:bg-gray-700 dark:text-gray-300'
+    case 'g1-pro-tier': return 'bg-blue-100 text-blue-600 dark:bg-blue-900/40 dark:text-blue-300'
+    case 'g1-ultra-tier': return 'bg-purple-100 text-purple-600 dark:bg-purple-900/40 dark:text-purple-300'
+    default: return ''
+  }
+}
+
 // All available columns
 const allColumns = computed(() => {
   const c = [