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/utils/feed-fetcher.ts

@@ -9,7 +9,6 @@ export interface FeedData {
 export async function fetchFeedData(
   feedId: string,
   env: Env,
-  feedPath: "rss" | "atom",
 ): Promise<FeedData | null> {
   const storage = env.EMAIL_STORAGE;
 
@@ -26,8 +25,6 @@ export async function fetchFeedData(
   )) as FeedConfig | null) ?? {
     title: `Newsletter Feed ${feedId}`,
     description: "Converted email newsletter",
-    site_url: `https://${env.DOMAIN}/${feedPath}/${feedId}`,
-    feed_url: `https://${env.DOMAIN}/${feedPath}/${feedId}`,
     language: "en",
     created_at: Date.now(),
   };