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/main.py

@@ -2,6 +2,7 @@ from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
 
 from app.api import api_router
 from app.core.database import init_db
@@ -28,6 +29,15 @@ def create_app() -> FastAPI:
     """Create and configure the FastAPI application."""
     app = FastAPI(lifespan=lifespan)
 
+    # Add CORS middleware
+    app.add_middleware(
+        CORSMiddleware,
+        allow_origins=["http://localhost:8001"],
+        allow_credentials=True,
+        allow_methods=["*"],
+        allow_headers=["*"],
+    )
+
     app.add_middleware(LoggingMiddleware)
 
     # Include API routes