Post-1175 cleanup: OpenTofu, requirements bump, valkey, TFC parallelism

[filippo-20tab]

Summary

Post-merge cleanup of the Next.js sub-bootstrapper aligned with Talos #286 and django-continuous-delivery #325.

Provisioning

• Bump Vault provider ~> 4.2.0 → ~> 5.0.
• Replace Terraform with OpenTofu; rename terraform/ dir → tofu/; runner invokes tofu instead of terraform.
• Serialize TFC workspace creation via -parallelism=1 to avoid sporadic 422s on tfe_project race.

Misc

• Bump base requirements (`click`, `cookiecutter`, etc).
• Rename redis → valkey across CLI, env vars, vars (frontend doesn't carry a Python redis client).

Test plan

• Bootstrap a Next.js service standalone and verify build/deploy
• CI pipeline runs at least up to deploy on a sandbox project