Add feature-usage tracking and dashboard charts

Instruments FeatureEvaluationService with System.Diagnostics.Metrics (Counter + IMeterFactory). Accumulates per-feature/day evaluation counts in-process, flushed every 30s to a new FeatureUsageDaily Postgres table via PostgreSQL ON CONFLICT upsert. Caches dashboard query results for 180s in IMemoryCache. New GET /api/v1/environments/{id}/usage endpoint returns top-10 features last day and 14-day per-day breakdown. Admin dashboard shows two ApexCharts bar charts: top features and evaluations by day with per-flag hover tooltip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/admin/package-lock.json

@@ -12,10 +12,12 @@
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         "@iconify/vue": "^4.1.0",
         "@vueuse/core": "^11.0.0",
+        "apexcharts": "^5.15.0",
         "axios": "^1.15.0",
         "pinia": "^3.0.4",
         "vue": "^3.4.0",
         "vue-router": "^4.3.0",
+        "vue3-apexcharts": "^1.11.1",
         "vue3-perfect-scrollbar": "^2.0.0",
         "vuetify": "^3.7.5"
       },
@@ -4281,6 +4283,11 @@
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       }
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       "license": "MIT"
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+    "node_modules/vue3-apexcharts": {
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+        "vue": ">=3.0.0"
+      },
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+        "apexcharts": {
+          "optional": false
+        }
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