feat: add reusable Renovate config validation workflow

New workflow_call workflow (.gitea/workflows/validate.yaml) that a consumer
repo can call to statically validate its renovate.json with
renovate-config-validator inside the official renovate/renovate image.

What it catches before Renovate runs (avoiding the dashboard-freeze class of
bug seen when an invalid config makes every extract fail):
- schema violations / unknown fields
- forbidden fields in customManagers (e.g. registryUrlsTemplate -
  registryUrls is the only allowed form)
- malformed packageRules / matchStrings regexes
- bad extends / ignorePresets references

It does NOT run Renovate and does NOT touch dependencies - just parses the
config. Non-blocking when the caller has no renovate.json.

Usage in a repo (.gitea/workflows/renovate.yaml or a dedicated workflow):

    jobs:
      validate:
        uses: perso/gitea-pipelines/.gitea/workflows/validate.yaml@main

.gitea/workflows/validate.yaml

@@ -0,0 +1,32 @@
+name: Renovate Config Validation
+
+on:
+  workflow_call:
+    inputs:
+      config_path:
+        description: "Path to the Renovate config to validate (a renovate.json in the calling repo). Non-blocking when absent."
+        required: false
+        type: string
+        default: "renovate.json"
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    container: renovate/renovate:43
+    steps:
+      - name: Checkout caller repo
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
+
+      - name: Validate the repo's Renovate config
+        run: |
+          FILE="${{ inputs.config_path }}"
+          if [ ! -f "$FILE" ]; then
+            echo "No Renovate config at $FILE — skipping validation (this repo has no renovate.json)."
+            exit 0
+          fi
+          echo "Validating $FILE"
+          # renovate-config-validator parses the file against the official
+          # Renovate schema and catches: invalid fields, bad regexes in
+          # customManagers, forbidden *Template fields (e.g. registryUrlsTemplate),
+          # malformed packageRules, etc. It does NOT run Renovate itself.
+          renovate-config-validator "$FILE"