Bump alembic from 1.17.1 to 1.18.5 in /app/backend

Bumps [alembic](https://github.com/sqlalchemy/alembic) from 1.17.1 to 1.18.5.
- [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: 1.18.5
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

app/backend/pyproject.toml

@@ -4,7 +4,7 @@ version = "0.1.0"
 description = "Couchers.org backend application"
 requires-python = "==3.14.*"
 dependencies = [
-    "alembic>=1.17.1",
+    "alembic>=1.18.5",
     "babel>=2.18.0",
     "boto3>=1.43.1",
     "cachetools>=6.2.1",