ci: enable Dependabot for npm + GitHub Actions

Mirrors the etherpad-lite pattern: daily schedule, dev deps grouped,
limit 30 open PRs. No semver filtering — CI is the gate per house rules.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    versioning-strategy: "increase"
+    open-pull-requests-limit: 30
+    groups:
+      dev-dependencies:
+        dependency-type: "development"