feat: implement sensor data aggregation from PicoClaw nodes

## Description
Implement the core L2 gateway function: aggregate sensor data from multiple PicoClaw (L1) edge nodes.

## Requirements
- [ ] HTTP server to receive edge reports from PicoClaw nodes
- [ ] In-memory buffer for aggregating sensor readings
- [ ] Batch upload to MoltClaw (L3) on interval or threshold
- [ ] Offline queue when L3 is unreachable

## Skills
Python, asyncio, HTTP