Merge development: fix CORS for local dev on port 3001

Resolved conflict by keeping env var approach from main and updating
the default to include both localhost:3000 and localhost:3001.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/backend/app/main.py

@@ -10,7 +10,7 @@ Base.metadata.create_all(bind=engine)
 
 app = FastAPI(title="BTC Portfolio API")
 
-origins = os.getenv("ALLOWED_ORIGINS", "http://localhost:3000").split(",")
+origins = os.getenv("ALLOWED_ORIGINS", "http://localhost:3000,http://localhost:3001").split(",")
 
 app.add_middleware(
     CORSMiddleware,