refactor(domain): make FeedId circulate through the domain and repository

FeedId is now the type of Feed.id and of every single-feed method on
FeedRepository; callers wrap raw strings via FeedId.fromTrusted at the
repository boundary. String-medium operations (URLs, logs, JSON,
list registry, email keys) stay string. Drop the redundant
generateFeedId wrapper in favour of FeedId.generate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/domain/value-objects/feed-id.ts

@@ -16,6 +16,15 @@ export class FeedId {
     return match ? new FeedId(match[1]) : null;
   }
 
+  /**
+   * Wrap an id we already trust — a value we minted ourselves and round-tripped
+   * through our own links or KV keys (route params, the feed list, email keys).
+   * No validation: a wrong id simply misses in KV and 404s, exactly as before.
+   */
+  static fromTrusted(value: string): FeedId {
+    return new FeedId(value);
+  }
+
   static generate(): FeedId {
     const noun1 = nouns[Math.floor(Math.random() * nouns.length)];
     const noun2 = nouns[Math.floor(Math.random() * nouns.length)];