Add simple FastAPI backend

backend/src/main.py

@@ -0,0 +1,11 @@
+from fastapi import FastAPI
+
+app = FastAPI(title="HanchuESS Solar Backend API")
+
+@app.get("/", tags=["Root"])
+def root():
+    return {"message": "Welcome to the HanchuESS Solar Backend API!"}
+
+if __name__ == "__main__":
+    import uvicorn
+    uvicorn.run(app, host="0.0.0.0", port=8050)