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>

src/lib/cloudflare-email.ts

@@ -0,0 +1,39 @@
+import PostalMime from "postal-mime";
+import { Env } from "../types";
+import { processEmail } from "./email-processor";
+
+export async function handleCloudflareEmail(
+  message: ForwardableEmailMessage,
+  env: Env,
+  ctx: ExecutionContext,
+): Promise<void> {
+  try {
+    const email = await PostalMime.parse(message.raw);
+
+    const fromAddress = email.from?.address ?? message.from;
+    const from =
+      email.from?.name && email.from.address
+        ? `${email.from.name} <${email.from.address}>`
+        : fromAddress;
+
+    const headers: Record<string, string> = {};
+    for (const h of email.headers) {
+      headers[h.key] = h.value;
+    }
+
+    await processEmail(
+      {
+        toAddress: message.to,
+        from,
+        senders: [message.from],
+        subject: email.subject ?? "(no subject)",
+        content: email.html ?? email.text ?? "",
+        receivedAt: email.date ? new Date(email.date).getTime() : Date.now(),
+        headers,
+      },
+      env,
+    );
+  } catch (error) {
+    console.error("Error processing Cloudflare email:", error);
+  }
+}