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/types/index.ts

@@ -69,6 +69,23 @@ export interface FeedListItem {
   expires_at?: number; // Cached from FeedConfig to avoid per-feed KV reads
 }
 
+// Cumulative monitoring counters (persisted as a KV singleton)
+export interface Counters {
+  feeds_created: number;
+  feeds_deleted: number;
+  emails_received: number;
+  emails_rejected: number;
+  last_email_at?: string; // ISO 8601
+  last_feed_created_at?: string; // ISO 8601
+  first_seen?: string; // ISO 8601 — first time counters were written (instance start)
+}
+
+// Monitoring API response: persisted counters + live-computed values
+export interface StatsResponse extends Counters {
+  active_feeds: number;
+  websub_subscriptions_active: number;
+}
+
 // WebSub (PubSubHubbub) subscription configuration
 export interface WebSubSubscription {
   callbackUrl: string;