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/config.py

@@ -0,0 +1,51 @@
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    # Server
+    api_host: str = "0.0.0.0"
+    api_port: int = 8878
+
+    # NATS
+    nats_url: str = "nats://nats:4222"
+    nats_token: str = ""
+
+    # Service identity
+    service_name: str = "realsense-d435"
+    service_domain: str = "vision"
+
+    # Security
+    api_key: str = ""
+
+    # Storage
+    images_dir: str = "/app/images"
+
+    # RealSense stream config
+    camera_width: int = 640
+    camera_height: int = 480
+    camera_fps: int = 30
+    camera_auto_open: bool = True
+
+    # YOLO detector (optional — disabled if model file or onnxruntime missing)
+    yolo_onnx_path: str = "/app/models/best.onnx"
+    yolo_classes: str = ""
+    yolo_conf_default: float = 0.5
+
+    @property
+    def yolo_class_list(self) -> list[str]:
+        return [s.strip() for s in self.yolo_classes.split(",") if s.strip()]
+
+    # CORS (comma-separated string, parsed into list)
+    cors_origins: str = "http://localhost:5173,http://localhost,http://localhost:80"
+
+    @property
+    def cors_origins_list(self) -> list[str]:
+        return [s.strip() for s in self.cors_origins.split(",")]
+
+    model_config = {
+        "env_file": ".env",
+        "env_file_encoding": "utf-8",
+    }
+
+
+settings = Settings()