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.test.ts

@@ -0,0 +1,36 @@
+import { describe, it, expect } from "vitest";
+import { FeedId } from "./feed-id";
+
+describe("FeedId.parse", () => {
+  it("extracts the feed id from an inbound address", () => {
+    expect(FeedId.parse("river.castle.42@example.com")?.value).toBe(
+      "river.castle.42",
+    );
+  });
+
+  it("preserves the original casing of the local part", () => {
+    expect(FeedId.parse("River.Castle.42@example.com")?.value).toBe(
+      "River.Castle.42",
+    );
+  });
+
+  it("rejects malformed feed ids", () => {
+    expect(FeedId.parse("user@example.com")).toBeNull();
+    expect(FeedId.parse("notanemail")).toBeNull();
+    expect(FeedId.parse("river.castle.4@example.com")).toBeNull();
+    expect(FeedId.parse("river.castle.123@example.com")).toBeNull();
+  });
+});
+
+describe("FeedId.generate", () => {
+  it("produces the noun.noun.NN format", () => {
+    for (let i = 0; i < 50; i++) {
+      expect(FeedId.generate().value).toMatch(/^[a-z]+\.[a-z]+\.\d{2}$/);
+    }
+  });
+
+  it("round-trips through parse from an address", () => {
+    const id = FeedId.generate();
+    expect(FeedId.parse(`${id.value}@example.com`)?.value).toBe(id.value);
+  });
+});