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 <[email protected]>

src/lib/email-processor.test.ts

@@ -6,6 +6,7 @@ import {
   ProcessEmailInput,
   RawAttachment,
 } from "./email-processor";
+import { getCounters } from "../utils/stats";
 
 const VALID_FEED_ID = "apple.mountain.42";
 const VALID_TO = `${VALID_FEED_ID}@test.getmynews.app`;
@@ -467,3 +468,31 @@ describe("processEmail — attachments", () => {
     expect(mockR2._has(oldAttachmentId)).toBe(false);
   });
 });
+
+describe("processEmail — monitoring counters", () => {
+  it("increments emails_received and sets last_email_at on success", async () => {
+    const env = createMockEnv();
+    await env.EMAIL_STORAGE.put(
+      `feed:${VALID_FEED_ID}:config`,
+      JSON.stringify({}),
+    );
+
+    await processEmail(makeInput(), env as any);
+
+    const counters = await getCounters(env.EMAIL_STORAGE as any);
+    expect(counters.emails_received).toBe(1);
+    expect(counters.emails_rejected).toBe(0);
+    expect(counters.last_email_at).toBeDefined();
+  });
+
+  it("increments emails_rejected when validation fails", async () => {
+    const env = createMockEnv();
+
+    // No feed config → 404 rejection
+    await processEmail(makeInput(), env as any);
+
+    const counters = await getCounters(env.EMAIL_STORAGE as any);
+    expect(counters.emails_rejected).toBe(1);
+    expect(counters.emails_received).toBe(0);
+  });
+});