feat(dashboard): add per-user drill-down for group, model, and endpoint distributions

Click on a group name, model name, or endpoint name in the distribution
tables to expand and show per-user usage breakdown (requests, tokens,
actual cost, standard cost).

Backend: new GET /admin/dashboard/user-breakdown API with group_id,
model, endpoint, endpoint_type filters.
Frontend: clickable rows with expand/collapse sub-table in all three
distribution charts.

frontend/src/components/charts/ModelDistributionChart.vue

@@ -71,7 +71,7 @@
       <div class="h-48 w-48">
         <Doughnut :data="chartData" :options="doughnutOptions" />
       </div>
-      <div class="max-h-48 flex-1 overflow-y-auto">
+      <div class="max-h-64 flex-1 overflow-y-auto">
         <table class="w-full text-xs">
           <thead>
             <tr class="text-gray-500 dark:text-gray-400">
@@ -83,30 +83,43 @@
             </tr>
           </thead>
           <tbody>
-            <tr
-              v-for="model in displayModelStats"
-              :key="model.model"
-              class="border-t border-gray-100 dark:border-gray-700"
-            >
-              <td
-                class="max-w-[100px] truncate py-1.5 font-medium text-gray-900 dark:text-white"
-                :title="model.model"
+            <template v-for="model in displayModelStats" :key="model.model">
+              <tr
+                class="border-t border-gray-100 cursor-pointer transition-colors hover:bg-gray-50 dark:border-gray-700 dark:hover:bg-dark-700/40"
+                @click="toggleBreakdown('model', model.model)"
               >
-                {{ model.model }}
-              </td>
-              <td class="py-1.5 text-right text-gray-600 dark:text-gray-400">
-                {{ formatNumber(model.requests) }}
-              </td>
-              <td class="py-1.5 text-right text-gray-600 dark:text-gray-400">
-                {{ formatTokens(model.total_tokens) }}
-              </td>
-              <td class="py-1.5 text-right text-green-600 dark:text-green-400">
-                ${{ formatCost(model.actual_cost) }}
-              </td>
-              <td class="py-1.5 text-right text-gray-400 dark:text-gray-500">
-                ${{ formatCost(model.cost) }}
-              </td>
-            </tr>
+                <td
+                  class="max-w-[100px] truncate py-1.5 font-medium text-blue-600 hover:text-blue-800 dark:text-blue-400 dark:hover:text-blue-300"
+                  :title="model.model"
+                >
+                  <span class="inline-flex items-center gap-1">
+                    <svg v-if="expandedKey === `model-${model.model}`" class="h-3 w-3 shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M19 9l-7 7-7-7"/></svg>
+                    <svg v-else class="h-3 w-3 shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M9 5l7 7-7 7"/></svg>
+                    {{ model.model }}
+                  </span>
+                </td>
+                <td class="py-1.5 text-right text-gray-600 dark:text-gray-400">
+                  {{ formatNumber(model.requests) }}
+                </td>
+                <td class="py-1.5 text-right text-gray-600 dark:text-gray-400">
+                  {{ formatTokens(model.total_tokens) }}
+                </td>
+                <td class="py-1.5 text-right text-green-600 dark:text-green-400">
+                  ${{ formatCost(model.actual_cost) }}
+                </td>
+                <td class="py-1.5 text-right text-gray-400 dark:text-gray-500">
+                  ${{ formatCost(model.cost) }}
+                </td>
+              </tr>
+              <tr v-if="expandedKey === `model-${model.model}`">
+                <td colspan="5" class="p-0">
+                  <UserBreakdownSubTable
+                    :items="breakdownItems"
+                    :loading="breakdownLoading"
+                  />
+                </td>
+              </tr>
+            </template>
           </tbody>
         </table>
       </div>
@@ -193,7 +206,9 @@ import { useI18n } from 'vue-i18n'
 import { Chart as ChartJS, ArcElement, Tooltip, Legend } from 'chart.js'
 import { Doughnut } from 'vue-chartjs'
 import LoadingSpinner from '@/components/common/LoadingSpinner.vue'
-import type { ModelStat, UserSpendingRankingItem } from '@/types'
+import UserBreakdownSubTable from './UserBreakdownSubTable.vue'
+import type { ModelStat, UserSpendingRankingItem, UserBreakdownItem } from '@/types'
+import { getUserBreakdown } from '@/api/admin/dashboard'
 
 ChartJS.register(ArcElement, Tooltip, Legend)
 
@@ -213,6 +228,8 @@ const props = withDefaults(defineProps<{
   showMetricToggle?: boolean
   rankingLoading?: boolean
   rankingError?: boolean
+  startDate?: string
+  endDate?: string
 }>(), {
   enableRankingView: false,
   rankingItems: () => [],
@@ -226,6 +243,33 @@ const props = withDefaults(defineProps<{
   rankingError: false
 })
 
+const expandedKey = ref<string | null>(null)
+const breakdownItems = ref<UserBreakdownItem[]>([])
+const breakdownLoading = ref(false)
+
+const toggleBreakdown = async (type: string, id: string) => {
+  const key = `${type}-${id}`
+  if (expandedKey.value === key) {
+    expandedKey.value = null
+    return
+  }
+  expandedKey.value = key
+  breakdownLoading.value = true
+  breakdownItems.value = []
+  try {
+    const res = await getUserBreakdown({
+      start_date: props.startDate,
+      end_date: props.endDate,
+      model: id,
+    })
+    breakdownItems.value = res.users || []
+  } catch {
+    breakdownItems.value = []
+  } finally {
+    breakdownLoading.value = false
+  }
+}
+
 const emit = defineEmits<{
   'update:metric': [value: DistributionMetric]
   'ranking-click': [item: UserSpendingRankingItem]