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.

tests/test_services.py

@@ -0,0 +1,73 @@
+"""Smoke tests for the RealSense D435 service layer.
+
+These tests exercise the dataclasses + the driver in its un-initialized
+state. Tests that need real hardware are skipped at runtime.
+"""
+import pytest
+import numpy as np
+
+
+def _camera_module():
+    try:
+        return __import__("src.backend.services.camera", fromlist=["*"])
+    except ImportError as exc:
+        pytest.skip(f"pyrealsense2 not importable: {exc}")
+
+
+class TestCameraIntrinsics:
+    def test_creation(self):
+        mod = _camera_module()
+        i = mod.CameraIntrinsics(
+            fx_ir=383.0, fy_ir=383.0, cx_ir=320.0, cy_ir=240.0,
+            fx_rgb=610.0, fy_rgb=610.0, cx_rgb=320.0, cy_rgb=240.0,
+        )
+        assert i.fx_ir == 383.0
+        assert i.fx_rgb == 610.0
+
+
+class TestCameraFrame:
+    def test_creation(self):
+        mod = _camera_module()
+        rgb = np.zeros((480, 640, 3), dtype=np.uint8)
+        depth = np.zeros((480, 640), dtype=np.uint16)
+        f = mod.CameraFrame(rgb=rgb, depth=depth, timestamp=1234.5)
+        assert f.rgb.shape == (480, 640, 3)
+        assert f.depth.shape == (480, 640)
+        assert f.timestamp == 1234.5
+
+
+class TestDiscoveredDevice:
+    def test_creation(self):
+        mod = _camera_module()
+        d = mod.DiscoveredDevice(index=0, vid=0x8086, pid=0x0B07, serial="ABC123", bus=1, device=5)
+        assert d.vid == 0x8086
+        assert d.serial == "ABC123"
+        assert d.index == 0
+
+
+class TestRealSenseD435:
+    def test_defaults(self):
+        mod = _camera_module()
+        cam = mod.RealSenseD435()
+        assert not cam.is_open()
+        assert not cam.is_initialized()
+        assert cam.intrinsics is None
+
+    def test_capture_when_closed(self):
+        mod = _camera_module()
+        cam = mod.RealSenseD435()
+        assert cam.capture() is None
+        assert cam.get_rgb() is None
+        assert cam.get_depth() is None
+
+    def test_discover_requires_init(self):
+        mod = _camera_module()
+        cam = mod.RealSenseD435()
+        with pytest.raises(RuntimeError, match="not initialized"):
+            cam.discover()
+
+    def test_open_requires_init(self):
+        mod = _camera_module()
+        cam = mod.RealSenseD435()
+        # open() returns False rather than raising when not initialized.
+        assert cam.open(0) is False