[Feature Request] Automate `backend.tf` state bucket substitutions

## Feature Description
Automate the injection of state bucket names into `backend.tf` files.

## Use Case
During the bootstrap phase, developers are currently forced to run manual command-line `sed` replacements to swap out `UPDATE_ME` placeholders with actual bucket names.

## Proposed Solution
Develop a lightweight Go or Bash pre-flight script to handle variable substitution in `backend.tf` files dynamically before `terraform init` is run.

## Compliance & Deployment Context
* **Target Deployment Type(s):**
* [ ] US Region Restricted (e.g., Access Policy constraint)
* [ ] FedRAMP Medium
* [ ] FedRAMP High
* [ ] DoD IL4
* [ ] DoD IL5
* [x] All / General

## Reusability Check
Stellar Engine prioritizes reusability.
* [x] I have checked if this functionality can be achieved by extending an existing module or blueprint.
* [x] I have verified that this does not duplicate existing functionality.

## Alternatives Considered
Keeping the manual `sed` instructions in the README, which leads to user error.

## Additional Context
This was explicitly requested in the legacy Feature Braindump from 2023.