chore(deps): bump black in /backend in the uv group across 1 directory

Bumps the uv group with 1 update in the /backend directory: [black](https://github.com/psf/black).


Updates `black` from 26.1.0 to 26.3.1
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](https://github.com/psf/black/compare/26.1.0...26.3.1)

---
updated-dependencies:
- dependency-name: black
  dependency-version: 26.3.1
  dependency-type: direct:production
  dependency-group: uv
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/pyproject.toml

@@ -33,7 +33,7 @@ dependencies = [
 [project.optional-dependencies]
 dev = [
     "aiosqlite==0.22.1",
-    "black==26.1.0",
+    "black==26.3.1",
     "coverage[toml]==7.13.4",
     "flake8==7.3.0",
     "httpx==0.28.1",