ci: enable Dependabot for uv and GitHub Actions (#770)

Adds .github/dependabot.yml with weekly update checks for Python
dependencies (pyproject.toml / uv.lock via the uv ecosystem) and
GitHub Actions workflow pins. Updates are grouped into a single PR
per ecosystem to keep review noise low.

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

Author: dbrattli

.github/dependabot.yml

@@ -0,0 +1,21 @@
+version: 2
+updates:
+  - package-ecosystem: "uv"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10
+    groups:
+      python-dependencies:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    groups:
+      github-actions:
+        patterns:
+          - "*"