fix: Use meaningful plugin activity dates

[freddiewrites]

Summary

• Replace repository-wide pushed_at with the latest meaningful plugin activity from a distributable release, appcast, or code and packaging commit.
• Ignore documentation-only commits, preserve the previous meaningful date on failures, and skip deep inspection when a repository push has not changed.
• Add source and push metadata, batched GraphQL lookups, and a rate-limited backfill path for existing records.
• Keep the test suite focused at five cases, including the Turkish Data README-only warning regression.

Testing

• ruby -Itest test/plugin-recency-test.rb — 5 runs, 13 assertions
• ruby -c lib/plugin-recency.rb
• ruby -c Rakefile
• git diff --check

Rollout

Run the backfill workflow in batches and audit the first 50 results before merging. Merge this PR before the matching Website-frontend change so the next import uses corrected activity dates.