feat: add basic UI2CB and CB2UI communication

The Python application exposes an endpoint /message for the UI to send
messages to.

It also exposes an SSE endpoint /sse for the UI to listen to. Every
second, the CB sends the current time to UI.

ref: N25B-107
ref: N25B-110

main.py

@@ -1,6 +1,39 @@
-def main():
-    print("Hello from pepperplus-cb!")
+import asyncio
+from fastapi import FastAPI, Request
+from fastapi.middleware.cors import CORSMiddleware
 
+from fastapi.responses import StreamingResponse
+from pydantic import BaseModel
+import datetime
+from sse_starlette import EventSourceResponse
 
-if __name__ == "__main__":
-    main()
+class Message(BaseModel):
+    message: str
+
+app = FastAPI()
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["http://localhost:5173"],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+@app.post("/message")
+async def receive_message(message: Message):
+    print(f"Received message: {message}")
+    return { "status": "Message received" }
+
+@app.get("/sse")
+async def sse_endpoint(request: Request):
+    async def event_generator():
+        while True:
+            if await request.is_disconnected():
+                break
+            
+            current_time = datetime.datetime.now().strftime("%H:%M:%S")
+            yield f"data: Server time: {current_time}\n\n"
+            await asyncio.sleep(1)
+
+    return EventSourceResponse(event_generator())