chore: add dependabot config

Add Dependabot for the three supported ecosystems in this repo:
GitHub Actions, Terraform (root + grafana/hetzner/pagerduty modules),
and Docker (service compose files + dotfile Dockerfiles). Weekly
schedule with per-ecosystem grouping to keep PR noise down.

.github/dependabot.yml

@@ -0,0 +1,37 @@
+version: 2
+updates:
+  # GitHub Actions used across all workflows
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    groups:
+      github-actions:
+        patterns:
+          - "*"
+
+  # Terraform root and provider modules
+  - package-ecosystem: terraform
+    directories:
+      - /terraform
+      - /terraform/grafana
+      - /terraform/hetzner
+      - /terraform/pagerduty
+    schedule:
+      interval: weekly
+    groups:
+      terraform:
+        patterns:
+          - "*"
+
+  # Docker images in service compose files and dotfile Dockerfiles
+  - package-ecosystem: docker
+    directories:
+      - /ansible/services/*
+      - /ansible/dotfiles
+    schedule:
+      interval: weekly
+    groups:
+      docker:
+        patterns:
+          - "*"