feat(helm): add support for external secrets in dependency-track deployment

[NoUseFreak]

Summary

• Add existingSecret support to the DependencyTrack deployment, allowing database credentials (host, username, password) and admin password to be sourced from a pre-existing Kubernetes Secret instead of the chart-managed one
• Previously, dtrack always read credentials from the chart-managed *-secrets Secret, which broke when using an external database with pre-provisioned secrets (e.g. from Terraform)

Test Checklist

• Helm template renders without errors
• Terraform validates/plans cleanly (unchanged)
• Manually verified on staging cluster (if applicable)
• Rollback strategy documented

Infrastructure

• Helm: helm template renders correctly (unchanged)
• Terraform: terraform validate passes (unchanged)
• Terraform: terraform plan shows expected changes (unchanged)
• ArgoCD: Application manifests are valid (unchanged)
• N/A - documentation only

[brandonrc]

Thanks for the PR. This branch has merge conflicts with main now, so it cannot be merged as-is. Could you rebase onto the latest main, resolve the conflicts, and push? CI will re-run afterward.

Heads up: the red SonarCloud Scan is a known non-blocking issue on fork PRs (GitHub withholds SONAR_TOKEN from forks; the job is continue-on-error), so you can ignore that one.

[github-actions]

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