chore(deps): bump alembic from 1.17.2 to 1.18.4 in /backend-api

Bumps [alembic](https://github.com/sqlalchemy/alembic) from 1.17.2 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>

Author: dependabot[bot]

backend-api/pyproject.toml

@@ -13,7 +13,7 @@ dependencies = [
     "fastapi-users[sqlalchemy]>=13.0.0",
     "sqlalchemy>=2.0.0",
     "asyncpg>=0.29.0",
-    "alembic>=1.13.0",
+    "alembic>=1.18.4",
     # OAuth helpers (Google SSO)
     "httpx>=0.27.0",
     "httpx-oauth>=0.15.0",