refactor: extract url helpers, add EMAIL_DOMAIN support

- Add src/utils/urls.ts with baseUrl, feedRssUrl, feedAtomUrl, feedUrl,
  feedEmailAddress, feedTopicPattern
- Add optional EMAIL_DOMAIN env var so web domain and email domain can
  differ (e.g. demo.kill-the.news serves feeds, @kill-the.news receives mail)
- Replace all inline domain template literals with the new helpers
- Remove unused site_url/feed_url fields from FeedConfig
- Remove unused feedPath param from fetchFeedData
- Extract verifyCallback() to deduplicate verifyAndStoreSubscription /
  verifyAndDeleteSubscription

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

src/types/index.ts

@@ -3,6 +3,7 @@ export interface Env {
   EMAIL_STORAGE: KVNamespace;
   ADMIN_PASSWORD: string;
   DOMAIN: string;
+  EMAIL_DOMAIN?: string;
   ATTACHMENT_BUCKET?: R2Bucket;
   FEED_MAX_SIZE_BYTES?: string;
   PROXY_TRUSTED_IPS?: string;
@@ -33,8 +34,6 @@ export interface FeedConfig {
   description?: string;
   allowed_senders?: string[];
   language: string;
-  site_url: string;
-  feed_url: string;
   author?: string;
   created_at: number;
   updated_at?: number;