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/routes/atom.test.ts

@@ -81,8 +81,6 @@ describe("Atom Feed Route", () => {
         JSON.stringify({
           title: "Atom Test Feed",
           description: "Integration test",
-          site_url: "https://test.getmynews.app/rss/test-feed-atom",
-          feed_url: "https://test.getmynews.app/rss/test-feed-atom",
           language: "en",
           created_at: 1700000000000,
         }),