feat: add Cloudflare Email Workers support alongside ForwardEmail

Both email providers now work in parallel on the same Worker:
- ForwardEmail: existing POST /api/inbound webhook (unchanged)
- Cloudflare Email Routing: native `email` handler using postal-mime

New files:
- src/lib/email-processor.ts  shared business logic (feed lookup,
  sender allowlist, KV storage) extracted from inbound.ts
- src/lib/cloudflare-email.ts  Cloudflare `email` handler; parses
  raw RFC 2822 email with postal-mime, delegates to processEmail()
- src/lib/email-processor.test.ts  9 unit tests
- src/lib/cloudflare-email.test.ts  5 integration tests

Also fixes pre-existing CORS 204 response: c.text("", 204) →
c.body(null, 204) to match Hono's EmptyStatusCode constraint.

To enable: configure Cloudflare Email Routing with a catch-all rule
`*@domain.com` pointing to this Worker.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -30,6 +30,7 @@
   "dependencies": {
     "feed": "^5.2.0",
     "hono": "^4.11.7",
+    "postal-mime": "^2.7.4",
     "zod": "^4.3.6"
   }
 }