Replace hardcoded Backstage URLs with centralized Terraform variables

[coderabbitai]

Description

Several files in the codebase contain hardcoded Backstage URLs that should reference the newly added centralized Terraform variables (backstage_app_config_base_url) to ensure consistency and easier maintenance.

Files requiring updates:

1. app/templates/opentofu-child-module-repository/template.yaml:98

   • Current: url: https://backstage.osinfra.io/docs/default/component/${{ parameters.component_id }}
   • Should use: variable reference

2. app/catalog/entities.yaml

   • Current: hardcoded URLs like https://sb.backstage.osinfra.io and https://backstage.osinfra.io
   • Should use: variable references

Context

This issue was identified during PR #205 review where centralized Terraform variables were added for Backstage configuration. Having multiple hardcoded URLs defeats the purpose of centralization and creates potential for configuration drift.

References

• PR: Refactor environment and authentication configuration for shared use #205
• Comment: Refactor environment and authentication configuration for shared use #205 (comment)

Requested by: @brettcurtis