chore(deps): bump celery from 5.6.2 to 5.6.3 in /backend

Bumps [celery](https://github.com/celery/celery) from 5.6.2 to 5.6.3.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/v5.6.3/Changelog.rst)
- [Commits](celery/celery@v5.6.2...v5.6.3)

---
updated-dependencies:
- dependency-name: celery
  dependency-version: 5.6.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -32,7 +32,7 @@ alembic==1.13.0
 
 # Task Queue (Redis + Celery)
 redis==7.1.0
-celery==5.6.2
+celery==5.6.3
 
 # AI Providers (Multi-Model Support)
 openai==2.11.0