Code Change Summarizer

This PR adds OAuth2 authentication to the user login system. The change
improves security by using industry-standard authentication and enables
single sign-on with external providers.

## Changes

### Added
- OAuth2 authentication flow
- User session management
- Login/logout endpoints

### Modified
- User model to include OAuth tokens
- Authentication middleware
- Database schema

### Fixed
- Session timeout not working correctly
- Memory leak in token refresh

## Breaking Changes

⚠️ **[Breaking change description]**

**Impact:** [Who/what is affected]

**Migration Guide:**
1. [Step 1]
2. [Step 2]

**Before:**
```[language]
// Old code

// New code

**Example:**
```markdown
## Breaking Changes

⚠️ **Authentication endpoint signature changed**

**Impact:** All API clients must update their authentication calls.

**Migration Guide:**
1. Update authentication endpoint from `/auth/login` to `/auth/oauth/login`
2. Include `provider` parameter in request body
3. Handle new response format with OAuth tokens

**Before:**
```javascript
POST /auth/login
{ "username": "user", "password": "pass" }

POST /auth/oauth/login
{ "username": "user", "password": "pass", "provider": "google" }

### 5. Add Technical Details

**Explain implementation approach:**
- High-level technical approach
- Key algorithms or patterns used
- Important implementation details

**Document design decisions:**
- Why this approach was chosen
- Alternatives considered
- Trade-offs made

**Note architecture changes:**
- New components or modules
- Changed relationships
- Updated data flow

**Example:**
```markdown
## Technical Details

### Implementation Approach
Implemented OAuth2 using the Authorization Code flow with PKCE for enhanced
security. The authentication flow is handled by a new `AuthService` that
manages token exchange and refresh.

### Key Design Decisions
- **OAuth2 over SAML:** Chose OAuth2 for better mobile support and simpler
  implementation
- **PKCE extension:** Added PKCE to protect against authorization code
  interception attacks
- **Token storage:** Store refresh tokens in secure HTTP-only cookies

### Architecture Changes
- Added new `AuthService` layer between controllers and OAuth provider
- Introduced `TokenManager` for token lifecycle management
- Updated database schema to store OAuth provider information

## Dependencies

### Added
- `passport@0.6.0` - OAuth2 authentication library
- `passport-google-oauth20@2.0.0` - Google OAuth2 strategy

### Updated
- `express@4.17.1` → `express@4.18.2` - Security patches and bug fixes
- `jsonwebtoken@8.5.1` → `jsonwebtoken@9.0.0` - Updated for Node 18 support

### Removed
- `bcrypt@5.0.1` - Replaced by OAuth2, no longer needed for password hashing

## Testing

### How to Test
1. [Setup step]
2. [Action to perform]
3. [Expected result]
4. [Edge case to verify]

### Test Coverage
- Added [X] unit tests
- Added [Y] integration tests
- Current coverage: [Z]%

### Manual Testing Checklist
- [ ] Test scenario 1
- [ ] Test scenario 2
- [ ] Test edge case 1

## Testing

### How to Test
1. Start the application: `npm start`
2. Navigate to `/login`
3. Click "Sign in with Google"
4. Complete OAuth flow in popup
5. Verify you're redirected back and logged in
6. Check that session persists after page refresh

### Test Coverage
- Added 15 unit tests for AuthService
- Added 8 integration tests for OAuth flow
- Current coverage: 87% (up from 82%)

### Manual Testing Checklist
- [ ] Google OAuth login works
- [ ] Session persists across page refreshes
- [ ] Logout clears session correctly
- [ ] Token refresh works when token expires
- [ ] Error handling for failed OAuth

## Security Considerations

### Improvements
- ✅ Implemented PKCE to prevent authorization code interception
- ✅ Store refresh tokens in HTTP-only cookies to prevent XSS
- ✅ Added rate limiting on authentication endpoints
- ✅ Validate OAuth state parameter to prevent CSRF

### Security Review
- [ ] Security team review pending
- [ ] Penetration testing scheduled

## Performance Impact

### Expected Changes
- Login time: ~500ms (OAuth redirect adds latency)
- Token validation: <10ms (cached in memory)
- Database queries: +2 per login (OAuth token storage)

### Optimizations
- Implemented token caching to reduce database hits
- Added connection pooling for OAuth provider requests

## Architecture Notes

### Patterns Used
- **Strategy Pattern:** Different OAuth providers (Google, GitHub, etc.)
- **Factory Pattern:** Token creation and validation
- **Middleware Pattern:** Authentication checks

### Integration Points
- Integrates with existing User model
- Hooks into session management middleware
- Compatible with existing authorization system

## Documentation

- [ ] Updated README with OAuth setup instructions
- [ ] Added API documentation for new endpoints
- [ ] Updated environment variables guide
- [ ] Added OAuth provider configuration guide
- [ ] Updated changelog

## Related Issues

Closes #123
Fixes #456
Related to #789