fix: /api/capture no longer moves the frame /api/rgb and /api/depth share

The 100 ms cache of get_rgb/get_depth/cached_frame served the latest
aligned frame, so an /api/capture from another client between /api/rgb and
/api/depth made the depth come from a newer frame than the color (with two
clients on PC2, the multi-camera service and the pestle detector, this can
happen). Older than R01: capture() already refreshed _last_frame then.
The cache is now its own slot, renewed only by the cached readers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/backend/services/camera.py

@@ -151,6 +151,8 @@ class RealSenseD435:
         self._raw: Optional[FrameAcquisition[RawFrameset]] = None
         self._aligned: Optional[FrameAcquisition[CameraFrame]] = None
         self._last_aligned_raw_seq = 0
+        # The frame get_rgb/get_depth/cached_frame share for frame_cache_ms (see _cached_or_capture()).
+        self._cached: Optional[CameraFrame] = None
         self._color_available: bool = True
         self._color_format = rs.format.bgr8
         self._usb_type: str = "unknown"
@@ -364,6 +366,7 @@ class RealSenseD435:
         for thread in threads:
             thread.join(STOP_JOIN_S)
         self.device_index = None
+        self._cached = None
         logger.info("RealSense camera closed")
 
     def _start_acquisition(self) -> None:
@@ -554,13 +557,19 @@ class RealSenseD435:
         return CameraFrame(rgb=rgb, depth=depth_mm, timestamp=time.time())
 
     def _cached_or_capture(self) -> Optional[CameraFrame]:
-        """The latest ALIGNED frame if younger than frame_cache_ms, else the next one."""
-        aligned = self._aligned
-        if aligned is not None:
-            _, last = aligned.latest()
-            if last is not None and (time.time() - last.timestamp) * 1000.0 < self._frame_cache_ms:
-                return last
-        return self.capture()
+        """The frame the cached readers share if younger than frame_cache_ms, else the next aligned one.
+
+        Only these readers renew it: a capture() in between (e.g. /api/capture
+        from another client) does not move it, so /api/rgb then /api/depth
+        within frame_cache_ms return color and depth of ONE frame.
+        """
+        cached = self._cached
+        if cached is not None and (time.time() - cached.timestamp) * 1000.0 < self._frame_cache_ms:
+            return cached
+        frame = self.capture()
+        if frame is not None:
+            self._cached = frame
+        return frame
 
     def cached_frame(self) -> Optional[CameraFrame]:
         """Like get_rgb()/get_depth(), but the whole frame (so its encodings can be shared)."""