fix: declare provider requirement in the artifactapi module

pre-commit runs `tofu validate`/tflint on modules/artifactapi standalone, where
no required_providers was declared, so OpenTofu inferred hashicorp/artifactapi
and failed (and tflint flagged missing required_version/version constraint).

- add modules/artifactapi/versions.tf declaring required_version and the
  artifactapi provider (source = the registry, version >= 0.1.2)
- drop the now-duplicate required_providers from root.hcl's generated backend.tf;
  keep the provider config block + backend. Declaring it in both the module and
  the generated root would be a "Duplicate required providers" error at runtime.

Also revert the earlier init lock-cleanup: the CI failure was stale provider
references in state (fixed with `tofu state replace-provider`), not the lock.

modules/artifactapi/versions.tf

@@ -0,0 +1,9 @@
+terraform {
+  required_version = ">= 1.10"
+  required_providers {
+    artifactapi = {
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/artifactapi"
+      version = ">= 0.1.2"
+    }
+  }
+}