ci(collections): add weekly schedule trigger to url validation

validate-collection-urls.yml only ran on push/pull_request for
collections/**.json, so a URL going stale after merge (Kiwix rotating
a dated filename, openZIM renaming a resource) was never re-checked
until a user hit a 404. #127 proposed a weekly check for exactly this
reason; only the push/PR trigger ended up shipping.

No change to the validation logic, which already checks every URL
correctly via a range request (hardened in #1188). This only adds a
weekly cron trigger alongside the existing ones.

.github/workflows/validate-collection-urls.yml

@@ -7,6 +7,8 @@ on:
   pull_request:
     paths:
       - 'collections/**.json'
+  schedule:
+    - cron: '17 6 * * 1'
 
 jobs:
   validate-urls: