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/models/api.py

@@ -0,0 +1,54 @@
+from pydantic import BaseModel
+from typing import Any
+
+
+class ApiError(BaseModel):
+    code: str
+    message: str
+
+
+class ApiResponse(BaseModel):
+    success: bool
+    data: Any | None = None
+    error: ApiError | None = None
+
+
+def ok(data: Any) -> dict:
+    return {"success": True, "data": data, "error": None}
+
+
+def fail(code: str, message: str) -> dict:
+    return {"success": False, "data": None, "error": {"code": code, "message": message}}
+
+
+class DeviceInfo(BaseModel):
+    index: int
+    vid: str
+    pid: str
+    serial: str
+    bus: int
+    device: int
+
+
+class IntrinsicsInfo(BaseModel):
+    fx: float
+    fy: float
+    cx: float
+    cy: float
+
+
+class CameraIntrinsicsInfo(BaseModel):
+    ir: IntrinsicsInfo
+    rgb: IntrinsicsInfo
+
+
+class FrameInfo(BaseModel):
+    frame_id: int
+    timestamp: float
+    rgb_shape: list[int]
+    depth_shape: list[int]
+    depth_min_mm: int
+    depth_max_mm: int
+    depth_valid_pixels: int
+    rgb_jpeg_b64: str | None = None
+    depth_png_b64: str | None = None