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/discovery.py

@@ -0,0 +1,52 @@
+import json
+from nats.aio.client import Client
+from src.backend.config import settings
+from src.backend.nats_client.subjects import Subjects
+
+CAPABILITIES = {
+    "service": settings.service_name,
+    "domain": settings.service_domain,
+    "version": "1.0.0",
+    "description": "Intel RealSense D4xx stereo depth camera server",
+    "endpoints": {
+        "rest": {
+            "health": {"method": "GET", "path": "/api/health"},
+            "discover": {"method": "GET", "path": "/api/discover"},
+            "open": {"method": "POST", "path": "/api/open"},
+            "close": {"method": "POST", "path": "/api/close"},
+            "capture": {"method": "GET", "path": "/api/capture"},
+            "rgb": {"method": "GET", "path": "/api/rgb"},
+            "depth": {"method": "GET", "path": "/api/depth"},
+        },
+    },
+    "nats": {
+        "actions": [
+            {
+                "subject": Subjects.action("snapshot"),
+                "pattern": "request/reply",
+                "description": "Capture RGB+depth frame",
+                "request_schema": {"include_depth": "boolean"},
+                "response_schema": {"rgb_jpeg_b64": "string", "depth_png_b64": "string", "timestamp": "number"},
+            },
+            {
+                "subject": Subjects.action("status"),
+                "pattern": "request/reply",
+                "description": "Get camera status",
+                "response_schema": {"camera_open": "boolean", "sdk_initialized": "boolean"},
+            },
+        ],
+    },
+}
+
+
+async def publish_discovery(nc: Client):
+    await nc.publish(
+        Subjects.DISCOVERY_ANNOUNCE,
+        json.dumps(CAPABILITIES).encode(),
+    )
+
+    async def handle_discovery_request(msg):
+        await msg.respond(json.dumps(CAPABILITIES).encode())
+
+    await nc.subscribe(Subjects.DISCOVERY_REQUEST, cb=handle_discovery_request)
+    await nc.subscribe(Subjects.SERVICE_DISCOVERY, cb=handle_discovery_request)