Fix remote-stack log streaming: URL-encode agent token in WS proxy (0.13.2)

The agent-logs WebSocket proxy injected the agent token raw into the upstream
query string (?token=<token>). Tokens containing base64/url-special characters
(+, /, =) were then mangled by the query parser on the agent side (e.g. "+"
decoded to a space), so the agent rejected the stream with close code 4401 even
though the same token works for the HTTP API (where it travels in the
Authorization header). URL-encode the token with urllib.parse.quote so it
round-trips intact.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -55,7 +55,7 @@ async def lifespan(app: FastAPI):
     schedule_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.13.1", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.13.2", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,