chore(deps): bump alembic from 1.13.1 to 1.18.4 in /backend

Bumps [alembic](https://github.com/sqlalchemy/alembic) from 1.13.1 to 1.18.4.
- [Release notes](https://github.com/sqlalchemy/alembic/releases)
- [Changelog](https://github.com/sqlalchemy/alembic/blob/main/CHANGES)
- [Commits](https://github.com/sqlalchemy/alembic/commits)

---
updated-dependencies:
- dependency-name: alembic
  dependency-version: 1.18.4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -6,7 +6,7 @@ pydantic-settings==2.13.1
 
 # Database
 sqlalchemy==2.0.48  # Updated: Fixed Python 3.14 incompatibility with TypingOnly assertion (was 2.0.25)
-alembic==1.13.1
+alembic==1.18.4
 psycopg2-binary==2.9.11
 asyncpg==0.31.0