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.

app/api/v1/oauth.py

@@ -0,0 +1,111 @@
+"""OAuth2 authentication endpoints."""
+
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, HTTPException, Query, Response, status
+from fastapi.responses import RedirectResponse
+
+from app.core.config import settings
+from app.core.dependencies import get_auth_service, get_oauth_service
+from app.core.logging import get_logger
+from app.services.auth import AuthService
+from app.services.oauth import OAuthService
+
+router = APIRouter()
+logger = get_logger(__name__)
+
+
+@router.get("/{provider}/authorize")
+async def oauth_authorize(
+    provider: str,
+    oauth_service: Annotated[OAuthService, Depends(get_oauth_service)],
+) -> dict[str, str]:
+    """Get OAuth authorization URL."""
+    try:
+        # Generate secure state parameter
+        state = oauth_service.generate_state()
+
+        # Get authorization URL
+        auth_url = oauth_service.get_authorization_url(provider, state)
+
+    except HTTPException:
+        raise
+    except Exception as e:
+        logger.exception("OAuth authorization failed for provider: %s", provider)
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail="OAuth authorization failed",
+        ) from e
+    else:
+        return {
+            "authorization_url": auth_url,
+            "state": state,
+        }
+
+
+@router.get("/{provider}/callback")
+async def oauth_callback(
+    provider: str,
+    response: Response,
+    code: Annotated[str, Query()],
+    oauth_service: Annotated[OAuthService, Depends(get_oauth_service)],
+    auth_service: Annotated[AuthService, Depends(get_auth_service)],
+) -> RedirectResponse:
+    """Handle OAuth callback."""
+    try:
+        # Handle OAuth callback and get user info
+        oauth_user_info = await oauth_service.handle_callback(provider, code)
+
+        # Perform OAuth login (link or create user)
+        auth_response = await auth_service.oauth_login(oauth_user_info)
+
+        # Create and store refresh token
+        user = await auth_service.get_current_user(auth_response.user.id)
+        refresh_token = await auth_service.create_and_store_refresh_token(user)
+
+        # Set HTTP-only cookies for both tokens
+        response.set_cookie(
+            key="access_token",
+            value=auth_response.token.access_token,
+            max_age=auth_response.token.expires_in,
+            httponly=True,
+            secure=settings.COOKIE_SECURE,
+            samesite=settings.COOKIE_SAMESITE,
+        )
+        response.set_cookie(
+            key="refresh_token",
+            value=refresh_token,
+            max_age=settings.JWT_REFRESH_TOKEN_EXPIRE_DAYS * 24 * 60 * 60,
+            httponly=True,
+            secure=settings.COOKIE_SECURE,
+            samesite=settings.COOKIE_SAMESITE,
+        )
+
+        logger.info(
+            "OAuth login successful for user: %s via %s",
+            auth_response.user.email,
+            provider,
+        )
+
+        # Redirect back to frontend after successful authentication
+        return RedirectResponse(
+            url="http://localhost:8001/?auth=success",
+            status_code=302,
+        )
+
+    except HTTPException:
+        raise
+    except Exception as e:
+        logger.exception("OAuth callback failed for provider: %s", provider)
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail="OAuth callback failed",
+        ) from e
+
+
+@router.get("/providers")
+async def get_oauth_providers() -> dict[str, list[str]]:
+    """Get list of available OAuth providers."""
+    return {
+        "providers": ["google", "github"],
+    }