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/config.py

@@ -1,5 +1,6 @@
 import logging
-from pydantic_settings import BaseSettings
+from pydantic import field_validator
+from pydantic_settings import BaseSettings, SettingsConfigDict
 
 logging.basicConfig(
     format='%(asctime)s - %(name)s - %(levelname)s - %(message)s',
@@ -14,9 +15,26 @@ class Settings(BaseSettings):
     port: int = 8025
     imap_port: int = 11143
     basic_password: str = ""
+    imap_tls_cert: str = ""
+    imap_tls_key: str = ""
+    imap_cache_size: int = 500
+    imap_http_timeout: float = 30.0
 
-    class Config:
-        env_file = ".env"
+    model_config = SettingsConfigDict(env_file=".env")
+
+    @field_validator("imap_cache_size")
+    @classmethod
+    def cache_size_positive(cls, v):
+        if v <= 0:
+            raise ValueError("imap_cache_size must be > 0")
+        return v
+
+    @field_validator("imap_http_timeout")
+    @classmethod
+    def timeout_positive(cls, v):
+        if v <= 0:
+            raise ValueError("imap_http_timeout must be > 0")
+        return v
 
 
 settings = Settings()