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.

scripts/entrypoint.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+# RealSense D4xx uses uvcvideo via the librealsense2 backend, so no driver
+# unbind is needed — unlike the Nuwa HP60C entrypoint.
+#
+# librealsense expects the udev rules to be present on the host. The rules
+# package is installed inside the image too (librealsense2-udev-rules) but
+# udev itself runs on the host; if the camera is not visible to lsusb the
+# host udev rules are likely missing or stale.
+
+set -e
+
+PORT=${API_PORT:-8878}
+
+echo "[entrypoint] Starting RealSense D4xx API server on port ${PORT}..."
+exec python3 -m uvicorn src.backend.main:app --host 0.0.0.0 --port "${PORT}"