fix: use direct in-cluster kubernetes auth instead of kubeconfig file

- providers.tf: use host + token + ca_crt from mounted service account secrets
- workflow: remove kubeconfig generation step (no longer needed)
- variables.tf: remove unused kubeconfig_path variable

This is the standard pattern for running terraform inside k8s pods.

.forgejo/workflows/terraform-apply.yml

@@ -40,30 +40,6 @@ jobs:
         run: terraform fmt -check -recursive terraform/
         continue-on-error: true
 
-      - name: Generate In-Cluster Kubeconfig
-        run: |
-          mkdir -p /tmp
-          cat > /tmp/kubeconfig << 'EOF'
-          apiVersion: v1
-          kind: Config
-          clusters:
-          - cluster:
-              certificate-authority: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
-              server: https://kubernetes.default:443
-            name: in-cluster
-          contexts:
-          - context:
-              cluster: in-cluster
-              user: terraform
-            name: in-cluster
-          current-context: in-cluster
-          users:
-          - name: terraform
-            user:
-              tokenFile: /var/run/secrets/kubernetes.io/serviceaccount/token
-          EOF
-          chmod 600 /tmp/kubeconfig
-
       - name: Configure AWS Credentials
         env:
           AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
@@ -82,7 +58,6 @@ jobs:
         env:
           AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
           AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
-          TF_VAR_kubeconfig_path: /tmp/kubeconfig
           TF_SKIP_CREDENTIALS_VALIDATION: "true"
           TF_SKIP_REGION_VALIDATION: "true"
           TF_SKIP_REQUESTING_ACCOUNT_ID: "true"
@@ -120,7 +95,6 @@ jobs:
         env:
           AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
           AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
-          TF_VAR_kubeconfig_path: /tmp/kubeconfig
           TF_SKIP_CREDENTIALS_VALIDATION: "true"
         run: |
           terraform plan -out=tfplan
@@ -130,7 +104,6 @@ jobs:
         env:
           AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}
           AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
-          TF_VAR_kubeconfig_path: /tmp/kubeconfig
           TF_SKIP_CREDENTIALS_VALIDATION: "true"
         run: |
           terraform apply -auto-approve tfplan