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/utils/stats.ts

@@ -0,0 +1,84 @@
+import { Counters, Env, StatsResponse } from "../types";
+import { STATS_KEY } from "../config/constants";
+import { logger } from "../lib/logger";
+import { listAllFeeds } from "../routes/admin/helpers";
+
+const EMPTY_COUNTERS: Counters = {
+  feeds_created: 0,
+  feeds_deleted: 0,
+  emails_received: 0,
+  emails_rejected: 0,
+};
+
+export async function getCounters(kv: KVNamespace): Promise<Counters> {
+  try {
+    const stored = (await kv.get(STATS_KEY, {
+      type: "json",
+    })) as Counters | null;
+    return { ...EMPTY_COUNTERS, ...(stored || {}) };
+  } catch (error) {
+    logger.error("Error reading counters", { error: String(error) });
+    return { ...EMPTY_COUNTERS };
+  }
+}
+
+/**
+ * Read-modify-write the counters singleton. KV has no atomic increment, so
+ * concurrent invocations can lose updates — accepted given KV's eventual
+ * consistency and this app's low volume (see email-processor.ts storeEmail).
+ * Never throws: counter failures must not break ingestion or admin flows.
+ */
+export async function bumpCounters(
+  kv: KVNamespace,
+  changes: Partial<Omit<Counters, "first_seen">>,
+): Promise<void> {
+  try {
+    const current = await getCounters(kv);
+
+    current.feeds_created += changes.feeds_created ?? 0;
+    current.feeds_deleted += changes.feeds_deleted ?? 0;
+    current.emails_received += changes.emails_received ?? 0;
+    current.emails_rejected += changes.emails_rejected ?? 0;
+    if (changes.last_email_at) current.last_email_at = changes.last_email_at;
+    if (changes.last_feed_created_at)
+      current.last_feed_created_at = changes.last_feed_created_at;
+    if (!current.first_seen) current.first_seen = new Date().toISOString();
+
+    await kv.put(STATS_KEY, JSON.stringify(current));
+  } catch (error) {
+    logger.error("Error updating counters", { error: String(error) });
+  }
+}
+
+export async function countKeysByPrefix(
+  kv: KVNamespace,
+  prefix: string,
+): Promise<number> {
+  let total = 0;
+  let cursor: string | undefined;
+  try {
+    do {
+      const listed = await kv.list({ prefix, cursor, limit: 1000 });
+      total += listed.keys.length;
+      cursor = listed.list_complete ? undefined : listed.cursor;
+    } while (cursor);
+  } catch (error) {
+    logger.error("Error counting keys", { prefix, error: String(error) });
+  }
+  return total;
+}
+
+export async function getStats(env: Env): Promise<StatsResponse> {
+  const kv = env.EMAIL_STORAGE;
+  const [counters, feeds, websubCount] = await Promise.all([
+    getCounters(kv),
+    listAllFeeds(kv),
+    countKeysByPrefix(kv, "websub:"),
+  ]);
+
+  return {
+    ...counters,
+    active_feeds: feeds.length,
+    websub_subscriptions_active: websubCount,
+  };
+}