refactor: modularize IMAP server with dual login, STARTTLS, and test suite (#859)

refactor: modularize IMAP server with fixes and E2E tests

- Modularize IMAP server into imap_server, imap_mailbox, imap_message,
  imap_http_client, parse_email, config, models
- Support dual login: JWT token and address+password via backend
- Add STARTTLS support with configurable TLS cert/key
- Fix FETCH/STORE returning UID instead of sequence number (RFC 3501)
- Implement IMessageFile.open() for correct BODY[] raw MIME delivery
- Add UIDNEXT to SELECT response via _cbSelectWork override
- Use per-restart UIDVALIDITY to force client resync
- Pass raw MIME to SimpleMessage for accurate RFC822.SIZE
- Fix SENT mailbox returning empty source
- Handle CREATE command gracefully for Thunderbird compatibility
- Add IMAP E2E tests: auth, LIST, SELECT, STATUS, FETCH, SEARCH,
  STORE, UID FETCH, BODY[] integrity, size, seq numbers, SENT mailbox
- Add SMTP E2E tests using nodemailer: send plain/HTML, auth failure,
  sendbox verification
- Add sendTestMail helper using admin/send_mail

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

smtp_proxy_server/imap_http_client.py

@@ -0,0 +1,69 @@
+import logging
+
+import httpx
+from twisted.internet import defer, threads
+
+from config import settings
+
+_logger = logging.getLogger(__name__)
+_logger.setLevel(logging.INFO)
+
+
+class BackendClient:
+    """Async HTTP client for IMAP backend communication.
+
+    All public methods return Deferred via deferToThread to avoid
+    blocking the Twisted reactor with synchronous HTTP calls.
+    """
+
+    def __init__(self, password: str):
+        self.password = password
+        self._client = httpx.Client(
+            base_url=settings.proxy_url,
+            headers={
+                "Authorization": f"Bearer {password}",
+                "x-custom-auth": settings.basic_password,
+                "Content-Type": "application/json",
+            },
+            timeout=settings.imap_http_timeout,
+        )
+
+    def _get_endpoint(self, mailbox_name: str) -> str:
+        if mailbox_name == "INBOX":
+            return "/api/mails"
+        elif mailbox_name == "SENT":
+            return "/api/sendbox"
+        raise ValueError(f"Unknown mailbox: {mailbox_name}")
+
+    def _sync_get_message_count(self, mailbox_name: str) -> int:
+        endpoint = self._get_endpoint(mailbox_name)
+        res = self._client.get(f"{endpoint}?limit=1&offset=0")
+        res.raise_for_status()
+        return res.json()["count"]
+
+    def _sync_get_messages(
+        self, mailbox_name: str, limit: int, offset: int
+    ) -> tuple[list[dict], int | None]:
+        """Fetch messages from backend.
+
+        Returns (results, count) where count is only valid when offset=0.
+        """
+        endpoint = self._get_endpoint(mailbox_name)
+        res = self._client.get(f"{endpoint}?limit={limit}&offset={offset}")
+        res.raise_for_status()
+        data = res.json()
+        count = data.get("count") if offset == 0 else None
+        return data["results"], count
+
+    def get_message_count(self, mailbox_name: str) -> defer.Deferred:
+        return threads.deferToThread(self._sync_get_message_count, mailbox_name)
+
+    def get_messages(
+        self, mailbox_name: str, limit: int, offset: int
+    ) -> defer.Deferred:
+        return threads.deferToThread(
+            self._sync_get_messages, mailbox_name, limit, offset
+        )
+
+    def close(self):
+        self._client.close()