♻️ Simplify CORS configuration (#2429)

## AI Disclaimer

Using Codex with gpt-5.6-sol. Reviewed manually.

backend/app/main.py

@@ -24,15 +24,13 @@ app = FastAPI(
     generate_unique_id_function=custom_generate_unique_id,
 )
 
-# Set all CORS enabled origins
-if settings.all_cors_origins:
-    app.add_middleware(
-        CORSMiddleware,
-        allow_origins=settings.all_cors_origins,
-        allow_credentials=True,
-        allow_methods=["*"],
-        allow_headers=["*"],
-    )
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=[settings.FRONTEND_HOST],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
 
 app.include_router(api_router, prefix=settings.API_V1_STR)
 app.frontend("/", directory=FRONTEND_DIR)