ci: add grouped security-only Dependabot config

open-pull-requests-limit: 0 disables version-update PRs; the security
group bundles Dependabot security fixes per ecosystem into one PR.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019MauCcvXYKN2n8wj8UYfcT

Author: lukisch

.github/dependabot.yml

@@ -0,0 +1,13 @@
+# Security-only: keine version-updates (limit 0), Security-Fixes gruppiert
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 0
+    groups:
+      security:
+        applies-to: security-updates
+        patterns:
+          - "*"