Bump opencv from 0.88.8 to 0.91.1 in /backend

Bumps [opencv](https://github.com/twistedfall/opencv-rust) from 0.88.8 to 0.91.1.
- [Changelog](https://github.com/twistedfall/opencv-rust/blob/master/CHANGES.md)
- [Commits](twistedfall/opencv-rust@v0.88.8...v0.91.1)

---
updated-dependencies:
- dependency-name: opencv
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/Cargo.lock

@@ -14,7 +14,7 @@ futures-timer = { version = "3.0.3" }
 itertools = { workspace = true }
 lapin = { version = "2.3.1" }
 ndarray = { version = "0.15.6" }
-opencv = { version = "0.88.8", default-features = false, features = [
+opencv = { version = "0.91.1", default-features = false, features = [
     "imgproc",
     "imgcodecs",
 ] }