Geo: Add calculator service and worker for job artifact verification summaries

Implement the calculator service and cron worker that maintain the verification summaries table.

Calculator service:

• Picks up to 100 dirty buckets (oldest first)
• Marks them as calculating
• Runs a single grouped COUNT query against ci_job_artifact_states for all buckets
• Upserts results back as clean with accurate counts

Calculator worker:

• Cron worker gated behind geo_job_artifact_verification_summaries feature flag
• Loops until all dirty buckets are processed or 3-minute runtime limit is reached
• Uses RuntimeLimiter and deduplicate :until_executed to prevent concurrent runs

On deploy: nothing changes. Worker exits immediately (FF disabled, no data to process).

Part of: #512646 (closed)