Add Web Vitals performance tracking (LCP, INP, CLS, FCP, TTFB)

End-to-end performance metrics tracking: dedicated database table with
percentile aggregation, tracker collection behind data-perf attribute,
/api/send handling, report API endpoints, and Performance dashboard page
with threshold-based metric cards, time-series chart, and per-page breakdown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/metrics/PerformanceCard.module.css

@@ -0,0 +1,19 @@
+.card {
+  transition: box-shadow 0.2s;
+}
+
+.card.selected {
+  box-shadow: 0 0 0 2px var(--base-color-primary);
+}
+
+.good .rating {
+  color: #0cce6b;
+}
+
+.needs-improvement .rating {
+  color: #ffa400;
+}
+
+.poor .rating {
+  color: #ff4e42;
+}