Initial commit: RealSense D435 API server

Drop-in replacement for the Nuwa-HP60C camera container, exposing RGB +
aligned depth + intrinsics via REST/WebSocket so the existing
pestello-pose-detector pipeline can consume frames without changes.

Key adaptations for D4xx on USB 2.0 (this robot has no USB 3 port exposed
externally — see STATUS.md):
- pyrealsense2 built from source; the librealsense v2.55.1 install rule
  doesn't ship the main .so, so we copy it from the build dir and add an
  __init__.py that re-exports the submodule.
- Color stream uses YUYV format on USB 2.x (BGR8/RGB8 time out on this
  bus); converted to BGR via cv2.cvtColor in capture().
- rs.align is skipped on USB 2 (incompatible with YUYV); color is
  cv2-resized to depth shape for approximate per-pixel alignment.
- Driver auto-detects usb_type_descriptor and falls back to the
  full-resolution BGR8 + rs.align pipeline when on USB 3.

Tested: /api/{health,status,discover,rgb,depth,depth/color,capture,
intrinsics} all return 200 with valid payloads at 640x480.

src/backend/nats_client/handlers.py

@@ -0,0 +1,66 @@
+import base64
+import json
+import logging
+
+from nats.aio.client import Client
+from src.backend.nats_client.subjects import Subjects
+
+logger = logging.getLogger(__name__)
+
+_camera = None
+
+
+def set_camera(camera):
+    global _camera
+    _camera = camera
+
+
+def _nats_response(success: bool, data=None, error=None) -> bytes:
+    return json.dumps({
+        "success": success,
+        "data": data,
+        "error": error,
+    }).encode()
+
+
+async def register_all(nc: Client):
+    async def handle_snapshot(msg):
+        try:
+            if not _camera or not _camera.is_open():
+                await msg.respond(_nats_response(
+                    False, error={"code": "NO_CAMERA", "message": "Camera not open"}
+                ))
+                return
+
+            import cv2
+            frame = _camera.capture()
+            if frame is None:
+                await msg.respond(_nats_response(
+                    False, error={"code": "TIMEOUT", "message": "Frame capture timeout"}
+                ))
+                return
+
+            _, rgb_buf = cv2.imencode(".jpg", frame.rgb, [cv2.IMWRITE_JPEG_QUALITY, 85])
+            _, depth_buf = cv2.imencode(".png", frame.depth)
+
+            await msg.respond(_nats_response(True, data={
+                "rgb_jpeg_b64": base64.b64encode(rgb_buf.tobytes()).decode(),
+                "depth_png_b64": base64.b64encode(depth_buf.tobytes()).decode(),
+                "timestamp": frame.timestamp,
+                "width": frame.rgb.shape[1],
+                "height": frame.rgb.shape[0],
+            }))
+        except Exception as e:
+            logger.error(f"Snapshot handler error: {e}")
+            await msg.respond(_nats_response(
+                False, error={"code": "PROCESSING_ERROR", "message": str(e)}
+            ))
+
+    async def handle_status(msg):
+        await msg.respond(_nats_response(True, data={
+            "camera_open": _camera.is_open() if _camera else False,
+            "sdk_initialized": _camera.is_initialized() if _camera else False,
+        }))
+
+    await nc.subscribe(Subjects.action("snapshot"), cb=handle_snapshot)
+    await nc.subscribe(Subjects.action("status"), cb=handle_status)