feat: add Renovate Bot configuration (#216)

Adds renovate.json to enable automated dependency updates for
pub (pubspec.yaml), Dockerfile, and Forgejo Actions workflows.
The github-actions manager fileMatch is extended to cover
.forgejo/workflows/ in addition to the default .github/ path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

renovate.json

@@ -0,0 +1,10 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended"
+  ],
+  "labels": ["dependencies"],
+  "github-actions": {
+    "fileMatch": ["^\\.forgejo/workflows/[^/]+\\.ya?ml$"]
+  }
+}