feat: Implement OAuth2 authentication with Google and GitHub

- Added OAuth2 endpoints for Google and GitHub authentication.
- Created OAuth service to handle provider interactions and user info retrieval.
- Implemented user OAuth repository for managing user OAuth links in the database.
- Updated auth service to support linking existing users and creating new users via OAuth.
- Added CORS middleware to allow frontend access.
- Created tests for OAuth endpoints and service functionality.
- Introduced environment configuration for OAuth client IDs and secrets.
- Added logging for OAuth operations and error handling.

tests/api/v1/test_oauth_endpoints.py

@@ -0,0 +1,151 @@
+"""Tests for OAuth authentication endpoints."""
+
+from typing import Any
+from unittest.mock import AsyncMock, patch
+
+import pytest
+from httpx import AsyncClient
+
+from app.services.oauth import OAuthUserInfo
+
+
+class TestOAuthEndpoints:
+    """Test OAuth API endpoints."""
+
+    @pytest.mark.asyncio
+    async def test_get_oauth_providers(self, test_client: AsyncClient) -> None:
+        """Test getting list of OAuth providers."""
+        response = await test_client.get("/api/v1/oauth/providers")
+
+        assert response.status_code == 200
+        data = response.json()
+        assert "providers" in data
+        assert "google" in data["providers"]
+        assert "github" in data["providers"]
+
+    @pytest.mark.asyncio
+    async def test_oauth_authorize_google(self, test_client: AsyncClient) -> None:
+        """Test OAuth authorization URL generation for Google."""
+        with patch("app.services.oauth.OAuthService.generate_state") as mock_state:
+            mock_state.return_value = "test_state_123"
+
+            response = await test_client.get("/api/v1/oauth/google/authorize")
+
+            assert response.status_code == 200
+            data = response.json()
+            assert "authorization_url" in data
+            assert "state" in data
+            assert data["state"] == "test_state_123"
+            assert "accounts.google.com" in data["authorization_url"]
+
+    @pytest.mark.asyncio
+    async def test_oauth_authorize_github(self, test_client: AsyncClient) -> None:
+        """Test OAuth authorization URL generation for GitHub."""
+        with patch("app.services.oauth.OAuthService.generate_state") as mock_state:
+            mock_state.return_value = "test_state_456"
+
+            response = await test_client.get("/api/v1/oauth/github/authorize")
+
+            assert response.status_code == 200
+            data = response.json()
+            assert "authorization_url" in data
+            assert "state" in data
+            assert data["state"] == "test_state_456"
+            assert "github.com" in data["authorization_url"]
+
+    @pytest.mark.asyncio
+    async def test_oauth_authorize_invalid_provider(
+        self, test_client: AsyncClient
+    ) -> None:
+        """Test OAuth authorization with invalid provider."""
+        response = await test_client.get("/api/v1/oauth/invalid/authorize")
+
+        assert response.status_code == 400
+        data = response.json()
+        assert "Unsupported OAuth provider" in data["detail"]
+
+    @pytest.mark.asyncio
+    async def test_oauth_callback_new_user(
+        self, test_client: AsyncClient, ensure_plans: tuple[Any, Any]
+    ) -> None:
+        """Test OAuth callback for new user creation."""
+        # Mock OAuth user info
+        mock_user_info = OAuthUserInfo(
+            provider="google",
+            provider_user_id="google_123",
+            email="newuser@gmail.com",
+            name="New User",
+            picture="https://example.com/avatar.jpg",
+        )
+
+        # Mock the entire handle_callback method to avoid actual OAuth API calls
+        with patch("app.services.oauth.OAuthService.handle_callback") as mock_callback:
+            mock_callback.return_value = mock_user_info
+
+            response = await test_client.get(
+                "/api/v1/oauth/google/callback",
+                params={"code": "auth_code_123", "state": "test_state"},
+                follow_redirects=False,
+            )
+
+            # OAuth callback should successfully process and redirect to frontend
+            assert response.status_code == 302
+            assert response.headers["location"] == "http://localhost:8001/?auth=success"
+
+            # The fact that we get a 302 redirect means the OAuth login was successful
+            # Detailed cookie testing can be done in integration tests
+
+    @pytest.mark.asyncio
+    async def test_oauth_callback_existing_user_link(
+        self, test_client: AsyncClient, test_user: Any, ensure_plans: tuple[Any, Any]
+    ) -> None:
+        """Test OAuth callback for linking to existing user."""
+        # Mock OAuth user info with same email as test user
+        mock_user_info = OAuthUserInfo(
+            provider="github",
+            provider_user_id="github_456",
+            email=test_user.email,  # Same email as existing user
+            name="Test User",
+            picture="https://github.com/avatar.jpg",
+        )
+
+        # Mock the entire handle_callback method to avoid actual OAuth API calls
+        with patch("app.services.oauth.OAuthService.handle_callback") as mock_callback:
+            mock_callback.return_value = mock_user_info
+
+            response = await test_client.get(
+                "/api/v1/oauth/github/callback",
+                params={"code": "auth_code_456", "state": "test_state"},
+                follow_redirects=False,
+            )
+
+            # OAuth callback should successfully process and redirect to frontend
+            assert response.status_code == 302
+            assert response.headers["location"] == "http://localhost:8001/?auth=success"
+
+            # The fact that we get a 302 redirect means the OAuth login was successful
+            # Detailed cookie testing can be done in integration tests
+
+    @pytest.mark.asyncio
+    async def test_oauth_callback_missing_code(self, test_client: AsyncClient) -> None:
+        """Test OAuth callback with missing authorization code."""
+        response = await test_client.get(
+            "/api/v1/oauth/google/callback",
+            params={"state": "test_state"},  # Missing code parameter
+        )
+
+        assert response.status_code == 422  # Validation error
+
+    @pytest.mark.asyncio
+    async def test_oauth_callback_invalid_provider(
+        self, test_client: AsyncClient
+    ) -> None:
+        """Test OAuth callback with invalid provider."""
+        response = await test_client.get(
+            "/api/v1/oauth/invalid/callback",
+            params={"code": "auth_code_123", "state": "test_state"},
+        )
+
+        assert response.status_code == 400
+        data = response.json()
+        assert "Unsupported OAuth provider" in data["detail"]