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/users.py

@@ -27,9 +27,9 @@ async def update_current_user_profile(
 ):
     """Update current user profile"""
     if user_update.email:
-        current_user.email = user_update.email
+        current_user.email = user_update.email  # type: ignore[assignment]
     if user_update.full_name:
-        current_user.full_name = user_update.full_name
+        current_user.full_name = user_update.full_name  # type: ignore[assignment]
 
     await db.commit()
     await db.refresh(current_user)