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/utils/feed-fetcher.ts

@@ -1,6 +1,5 @@
 import { Env, FeedConfig, FeedMetadata, EmailData } from "../types";
-
-const MAX_FEED_ITEMS = 20;
+import { MAX_FEED_ITEMS } from "../config/constants";
 
 export interface FeedData {
   feedConfig: FeedConfig;