Extract emails into separate package with clean interface

- Create packages/emails/ with EmailClient interface abstraction
- Wrap Postmark ServerClient in adapter for clean typing
- Add createLoggingEmailClient for dev mode (logs to console)
- Split email templates into individual files with full test coverage
- Update api-server to use new package via context injection
- Remove EMAIL_DEV_MODE - now uses POSTMARK_API_KEY presence
- Delete apps/api-server/src/utils/email.ts

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

packages/emails/src/client.test.ts

@@ -0,0 +1,46 @@
+import { describe, expect, it, mock } from "bun:test";
+import { createPostmarkClient } from "./client.js";
+
+const mockSendEmail = mock(() =>
+  Promise.resolve({ MessageID: "test-message-id-123" }),
+);
+
+void mock.module("postmark", () => ({
+  ServerClient: class MockServerClient {
+    sendEmail = mockSendEmail;
+  },
+}));
+
+describe("createPostmarkClient", () => {
+  it("should create an EmailClient with sendEmail method", () => {
+    const client = createPostmarkClient("test-api-key");
+    expect(typeof client.sendEmail).toBe("function");
+  });
+
+  it("should throw an error if API key is empty", () => {
+    expect(() => createPostmarkClient("")).toThrow(
+      "Postmark API key is required",
+    );
+  });
+
+  it("should convert our interface to Postmark format and return converted result", async () => {
+    const client = createPostmarkClient("test-api-key");
+
+    const result = await client.sendEmail({
+      from: "sender@example.com",
+      to: "recipient@example.com",
+      subject: "Test Subject",
+      htmlBody: "<p>HTML</p>",
+      textBody: "Text",
+    });
+
+    expect(mockSendEmail).toHaveBeenCalledWith({
+      From: "sender@example.com",
+      To: "recipient@example.com",
+      Subject: "Test Subject",
+      HtmlBody: "<p>HTML</p>",
+      TextBody: "Text",
+    });
+    expect(result.messageId).toBe("test-message-id-123");
+  });
+});