Add X (Twitter) OAuth integration

[tsahil01]

What this PR does:

• Adds "Continue with X" button to sign-in page (/session/new)
• Adds connect/disconnect X account to profile page (/users/me/profile)
• Implements OAuth 2.0 authentication flow

Key changes:

• OAuth setup: Added omniauth-twitter2 gem and configuration
• Database: Added provider and uid fields to users table with proper indexes
• UI: Added OAuth buttons to sign-in and profile pages
• Logic: OAuth callback handling, account linking, and error handling
• Environment: Updated .env.sample with required Twitter API credentials

Tests added:

• Model tests (test/models/user_oauth_test.rb): 18 tests covering OAuth connection, disconnection, validation, and edge cases
• Controller tests (test/controllers/sessions_oauth_test.rb): 9 tests for OAuth callback flows, error handling, and authentication scenarios
• System tests (test/system/oauth_integration_test.rb): End-to-end tests for complete OAuth user flows

How to test:

1. Set up Twitter app with OAuth 2.0 enabled
2. Add environment variables:

   TWITTER_CLIENT_ID=your_client_id
   TWITTER_CLIENT_SECRET=your_client_secret

3. Set callback URL in Twitter app: https://smallbets.com/auth/twitter2/callback
4. Test sign-in page and profile page OAuth flows

AI disclosure:

• Used claude-4-sonnet

Resolves: #20

[tsahil01]

@dvassallo can you review this one?