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/lib/email-processor.test.ts

@@ -220,8 +220,6 @@ describe("processEmail", () => {
       JSON.stringify({
         title: "Test",
         language: "en",
-        site_url: "https://example.com",
-        feed_url: `https://example.com/rss/${VALID_FEED_ID}`,
         created_at: Date.now(),
       }),
     );