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/domain.ts

@@ -0,0 +1,24 @@
+/**
+ * A normalised DNS domain (lowercased, no leading `@`, no trailing dots).
+ * Accepts both bare (`example.com`) and allowlist-style (`@example.com`) input.
+ */
+export class Domain {
+  private constructor(readonly value: string) {}
+
+  static parse(raw: string): Domain | null {
+    const normalized = raw
+      .trim()
+      .toLowerCase()
+      .replace(/^@+/, "")
+      .replace(/\.+$/, "");
+    return normalized ? new Domain(normalized) : null;
+  }
+
+  matches(other: Domain): boolean {
+    return this.value === other.value;
+  }
+
+  toString(): string {
+    return this.value;
+  }
+}