refactor(domain): add FeedId, EmailAddress and Domain value objects

Encapsulate the email/domain/feed-id parsing that was scattered as ad-hoc
regexes and split("@") calls into three small immutable value objects under
src/domain/value-objects/. EmailParser.extractFeedId and generateFeedId now
delegate to FeedId; the sender policy, favicon domain extraction and the admin
SenderField parse through EmailAddress/Domain.

Left as-is on purpose: forwardemail's multi-address free-text extraction and the
admin allow/block list normaliser, which operate on mixed email-or-domain input
that the single-address value objects would reject.

Behaviour-preserving; adds unit tests for each value object.

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

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

@@ -0,0 +1,29 @@
+import { nouns } from "../../data/nouns";
+
+// Feed IDs are noun1.noun2.XY (two lowercase nouns + a 2-digit suffix).
+const FEED_ID_IN_ADDRESS = /^([a-z]+\.[a-z]+\.\d{2})@/i;
+
+/**
+ * A feed identifier. `parse` pulls it from the local part of an inbound email
+ * address; `generate` mints a fresh one. The original casing is preserved.
+ */
+export class FeedId {
+  private constructor(readonly value: string) {}
+
+  /** Extract the feed id from an inbound address (`noun.noun.NN@domain`). */
+  static parse(emailAddress: string): FeedId | null {
+    const match = emailAddress.match(FEED_ID_IN_ADDRESS);
+    return match ? new FeedId(match[1]) : null;
+  }
+
+  static generate(): FeedId {
+    const noun1 = nouns[Math.floor(Math.random() * nouns.length)];
+    const noun2 = nouns[Math.floor(Math.random() * nouns.length)];
+    const number = Math.floor(Math.random() * 90) + 10;
+    return new FeedId(`${noun1}.${noun2}.${number}`);
+  }
+
+  toString(): string {
+    return this.value;
+  }
+}