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

.github/copilot-instructions.md

@@ -0,0 +1,31 @@
+# Copilot Instructions
+
+## Linting Requirements
+
+Before committing any changes, always run the relevant linters and fix all errors:
+
+### Backend (Python)
+
+```bash
+# Check code formatting
+black --check backend/
+
+# Lint with ruff
+ruff check backend/
+
+# Type check with mypy
+mypy backend/app --ignore-missing-imports
+```
+
+### Frontend (TypeScript/React)
+
+```bash
+cd frontend
+npm run lint
+```
+
+## Conventions
+
+- **Python**: Follow PEP 8. Use `black` for formatting. All ruff and mypy errors must be resolved before committing.
+- **TypeScript**: Follow the ESLint configuration. Avoid `any` types — use `unknown` with type narrowing instead. Remove unused variables and imports.
+- **SQLAlchemy**: Use `# type: ignore[assignment]` for Column attribute assignments and `# noqa: E712` for `== True` comparisons in SQLAlchemy queries (these are valid ORM patterns).