perf: eliminate N+1 queries across list views and add covering indexes

[eclectic-coding]

Closes #47

Summary

• Replace per-queue COUNT loop in QueuesController#index with a single GROUP BY queue_name aggregation — was O(N) queries, now always 2
• Combine two separate pluck calls for queue/priority filter options in JobsController#index into one query
• Replace 5+1 separate COUNT queries in CacheSizeStats#buckets with a single SUM(CASE WHEN ...) aggregation; derive #total from the already-computed bucket counts
• Add covering indexes to the dummy app schema: solid_queue_jobs (finished_at, created_at) for the slow-job scan; (queue_name, created_at) on solid_queue_scheduled_executions and solid_queue_blocked_executions (both previously lacked a queue-name index)
• Move install generator docs into the Installation section of the README

Test plan

• bundle exec rake passes (325 examples, 0 failures, no Rubocop offenses)
• Queues index renders correctly with multiple queues and paused state
• Jobs filter dropdowns populate queue and priority options correctly
• Cache size distribution buckets and total count display correctly

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