feat(monitoring): add stats counters API and public status page

Add GET /api/stats exposing cumulative counters (feeds created/deleted,
emails received/rejected, recent date-times) plus live values (active
feeds, active WebSub subscriptions). Counters persist in a stats:counters
KV singleton and are incremented at the email-processing chokepoint and
feed create/delete paths. Replace the / → /admin redirect with a public
status page rendering these figures with a link to the admin.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/styles/components.css

@@ -1010,3 +1010,36 @@ table.table code {
   opacity: 0.6;
   pointer-events: none;
 }
+
+/* Status page — stat cards grid */
+.stats-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
+  gap: var(--spacing-md);
+  margin-bottom: var(--spacing-lg);
+}
+
+.stat-card {
+  display: flex;
+  flex-direction: column;
+  gap: var(--spacing-xs);
+  margin-bottom: 0;
+}
+
+.stat-value {
+  font-size: 2rem;
+  font-weight: 700;
+  color: var(--color-primary);
+  line-height: 1.1;
+}
+
+.stat-value-time {
+  font-size: 1rem;
+  font-weight: 600;
+  color: var(--color-text-primary);
+}
+
+.stat-label {
+  font-size: 0.85rem;
+  color: var(--color-text-secondary);
+}