feat add remote hand tracking backend

backend/README.md

@@ -0,0 +1,73 @@
+# Hand Tracking Backend
+
+Remote-compatible Python backend for La-Fabrik hand tracking.
+
+The browser captures webcam frames, downsizes them, sends JPEG frames to this backend over WebSocket, and receives hand landmarks plus pinch state.
+
+## Setup
+
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r backend/requirements.txt
+python3 backend/download_model.py
+```
+
+## Run
+
+```bash
+python3 -m backend.main
+```
+
+The WebSocket endpoint is:
+
+```txt
+ws://localhost:8000/ws
+```
+
+## Health Check
+
+```txt
+http://localhost:8000/health
+```
+
+## Message Flow
+
+Client sends a compressed frame:
+
+```json
+{
+  "type": "frame",
+  "timestamp": 1234567890,
+  "width": 320,
+  "height": 240,
+  "image": "base64-jpeg"
+}
+```
+
+Server responds with detected hands:
+
+```json
+{
+  "type": "hands",
+  "timestamp": 1234567890,
+  "hands": [
+    {
+      "x": 0.5,
+      "y": 0.3,
+      "z": 0.1,
+      "handedness": "Right",
+      "isPinch": true,
+      "pinchDistance": 0.05,
+      "score": 0.92
+    }
+  ]
+}
+```
+
+## Notes
+
+- The backend does not read `cv2.VideoCapture(0)`.
+- This keeps local development and production behavior aligned.
+- Each browser connection sends its own webcam frames.
+- The backend rate-limits frames per connection and drops work when a client is already being processed.