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_api.py

@@ -0,0 +1,14 @@
+import pytest
+from httpx import AsyncClient, ASGITransport
+from src.backend.config import settings
+
+
+@pytest.mark.anyio
+async def test_health_without_api_key():
+    """Request without API key should fail with 403 when key is configured."""
+    from src.backend.main import app
+    transport = ASGITransport(app=app)
+    async with AsyncClient(transport=transport, base_url="http://test") as ac:
+        resp = await ac.get("/api/health")
+        if settings.api_key:
+            assert resp.status_code in (403, 422)