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-lock.json

@@ -11,6 +11,7 @@
       "dependencies": {
         "feed": "^5.2.0",
         "hono": "^4.11.7",
+        "postal-mime": "^2.7.4",
         "zod": "^4.3.6"
       },
       "devDependencies": {
@@ -2658,6 +2659,12 @@
         "url": "https://github.com/sponsors/jonschlinkert"
       }
     },
+    "node_modules/postal-mime": {
+      "version": "2.7.4",
+      "resolved": "https://registry.npmjs.org/postal-mime/-/postal-mime-2.7.4.tgz",
+      "integrity": "sha512-0WdnFQYUrPGGTFu1uOqD2s7omwua8xaeYGdO6rb88oD5yJ/4pPHDA4sdWqfD8wQVfCny563n/HQS7zTFft+f/g==",
+      "license": "MIT-0"
+    },
     "node_modules/postcss": {
       "version": "8.5.6",
       "resolved": "https://registry.npmjs.org/postcss/-/postcss-8.5.6.tgz",