feat: extract constants module (P2-10)

Centralises magic numbers and string literals into src/config/constants.ts
(FEED_MAX_BYTES, MAX_FEED_ITEMS, MAX_METADATA_EMAILS, ADMIN_COOKIE_MAX_AGE,
FORWARD_EMAIL_IPS_CACHE_TTL_MS, DEFAULT_LEASE_SECONDS, MAX_LEASE_SECONDS,
FEEDS_LIST_KEY). Consumers updated in storage.ts and feed-fetcher.ts.

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

src/config/constants.ts

@@ -0,0 +1,23 @@
+/** Maximum total size of emails stored per feed (bytes). */
+export const FEED_MAX_BYTES = 524288; // 512 KB
+
+/** Cache TTL for ForwardEmail.net IP list (milliseconds). */
+export const FORWARD_EMAIL_IPS_CACHE_TTL_MS = 24 * 60 * 60 * 1000; // 24 hours
+
+/** Admin session cookie max-age (seconds). */
+export const ADMIN_COOKIE_MAX_AGE = 60 * 60 * 24 * 7; // 1 week
+
+/** Maximum number of feed items exposed in RSS/Atom responses. */
+export const MAX_FEED_ITEMS = 20;
+
+/** Maximum number of email entries kept in feed metadata. */
+export const MAX_METADATA_EMAILS = 50;
+
+/** Default WebSub lease duration (seconds). */
+export const DEFAULT_LEASE_SECONDS = 86400; // 24 hours
+
+/** Maximum WebSub lease duration (seconds). */
+export const MAX_LEASE_SECONDS = 30 * 24 * 3600; // 30 days
+
+/** KV key for the global feed list. */
+export const FEEDS_LIST_KEY = "feeds:list";