Fix all mypy and eslint errors, add copilot lint instructions

Backend (mypy - 56 errors fixed):
- database.py: Fix async generator return type to AsyncGenerator
- database_models.py: Add type annotations for SQLEnum columns
- middleware.py: Use explicit Optional for exempt_paths parameter
- mail_processor.py: Fix type narrowing in fetch_emails, add Dict type annotation for KNOWN_PROVIDERS
- users.py, auth.py, providers.py, tasks.py: Add type: ignore comments for SQLAlchemy Column assignment patterns

Frontend (eslint - 3 errors, 2 warnings fixed):
- login/page.tsx: Replace any with unknown + type narrowing, prefix unused vars with underscore
- register/page.tsx: Replace any with unknown + type narrowing

Add .github/copilot-instructions.md with lint-check requirements

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/dac7ab78-fe27-4fe4-890f-32c6c1c6d881

backend/app/services/mail_processor.py

@@ -148,12 +148,12 @@ class MailProcessor:
         Fetch emails from the mail server.
         Returns list of raw email data.
         """
-        max_count = max_count or self.account.max_emails_per_check
+        effective_max: int = max_count if max_count is not None else self.account.max_emails_per_check  # type: ignore[assignment]
 
         if self.account.protocol in [MailProtocol.POP3, MailProtocol.POP3_SSL]:
-            return await self._fetch_pop3_emails(max_count)
+            return await self._fetch_pop3_emails(effective_max)
         else:
-            return await self._fetch_imap_emails(max_count)
+            return await self._fetch_imap_emails(effective_max)
 
     async def _fetch_pop3_emails(self, max_count: int) -> List[bytes]:
         """Fetch emails via POP3"""
@@ -394,7 +394,7 @@ class MailServerAutoDetect:
     """Auto-detect mail server settings based on email domain"""
 
     # Common mail server configurations
-    KNOWN_PROVIDERS = {
+    KNOWN_PROVIDERS: Dict[str, Dict[str, Any]] = {
         "gmail.com": {
             "name": "Gmail",
             "pop3_ssl": {"host": "pop.gmail.com", "port": 995},