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/api/v1/endpoints/providers.py

@@ -201,11 +201,11 @@ async def save_gmail_credential(
 
     if existing:
         # Update existing
-        existing.gmail_email = credential_in.gmail_email
-        existing.encrypted_access_token = encrypted_access
-        existing.encrypted_refresh_token = encrypted_refresh
-        existing.is_valid = True
-        existing.last_verified_at = datetime.utcnow()
+        existing.gmail_email = credential_in.gmail_email  # type: ignore[assignment]
+        existing.encrypted_access_token = encrypted_access  # type: ignore[assignment]
+        existing.encrypted_refresh_token = encrypted_refresh  # type: ignore[assignment]
+        existing.is_valid = True  # type: ignore[assignment]
+        existing.last_verified_at = datetime.utcnow()  # type: ignore[assignment]
         await db.commit()
         await db.refresh(existing)
         return existing