feat(email): forward non-feed mail to FALLBACK_FORWARD_ADDRESS

Lets you point a domain's catch-all at the worker without losing personal
mail: inbound mail that isn't a feed (invalid_address / feed_not_found) is
forwarded to an optional verified destination instead of being dropped.
Expired feeds and blocked senders are still dropped so newsletters never
leak to the fallback inbox. Unset env keeps the original drop-and-log path.

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

src/test/setup.ts

@@ -258,6 +258,7 @@ export const createMockEnv = (options: { withR2?: boolean } = {}) => ({
   EMAIL_STORAGE: new MockKV(),
   DOMAIN: "test.getmynews.app",
   ADMIN_PASSWORD: "test-password",
+  FALLBACK_FORWARD_ADDRESS: undefined as string | undefined,
   ...(options.withR2
     ? { ATTACHMENT_BUCKET: new MockR2() as unknown as R2Bucket }
     : {}),