feat(platform): switch dbs to argocd

[casey-brooks]

Summary

• replace platform, litellm, and agent-state DB StatefulSets/Services with postgres-helm Argo CD applications
• add postgres chart version variable and include DB apps in outputs/health checks
• refresh platform README and tfvars example for DB app deployment details

Testing

• NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform fmt -recursive
• NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform -chdir=stacks/platform init -input=false
• NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform -chdir=stacks/platform validate

Closes #64

[casey-brooks]

Test & Lint Summary

• Lint: NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform fmt -recursive (no errors)
• Tests:
  • NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform -chdir=stacks/platform init -input=false
  • NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform -chdir=stacks/platform validate
  • Result: 1 passed, 0 failed, 0 skipped

[noa-lucent]

Clean migration from Terraform-managed StatefulSets to ArgoCD-managed Helm applications. The PR correctly addresses all acceptance criteria from #64:

• All three DB StatefulSets/Services removed and replaced with argocd_application resources using oci://ghcr.io/agynio/charts/postgres-helm.
• Service DNS names preserved via fullnameOverride.
• wait = true + 5m timeouts for ordering guarantees.
• prune = false + self_heal = true for stateful safety.
• Downstream depends_on updated to reference new ArgoCD apps.
• Health checks, outputs, docs, and tfvars example updated consistently.

Left one minor comment about documenting the intentional divergence from the dynamic "automated" pattern. No major issues.

[casey-brooks]

Test & Lint Summary (update)

• Lint: NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform fmt -recursive (no errors)
• Tests:
  • NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform -chdir=stacks/platform init -input=false
  • NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#terraform -c terraform -chdir=stacks/platform validate
  • Result: 1 passed, 0 failed, 0 skipped

[noa-lucent]

Verified: inline comments added above all three DB sync_policy blocks. Prior feedback resolved. LGTM.