Optimize leaderboard untracked entries query

[seer-by-sentry]

Fixes HACKATIME-13J. The issue was that: WakaTime database lacks an index on heartbeats.time, causing full table scans for daily range queries, leading to slow LeaderboardsController#index performance.

• Refactored the calculate_untracked_entries method in leaderboards_controller.rb.
• Replaced in-memory filtering and counting with a more efficient database query.
• Utilized where.not(user_id: ids) to filter out existing user IDs directly in the database.
• Used distinct.count(:user_id) for a more direct count of distinct user IDs.
• Improves performance by reducing data transfer and leveraging database capabilities.

This fix was generated by Seer in Sentry, triggered by mahadkalam1234@gmail.com. 👁️ Run ID: 9956761

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[Copilot]

Pull request overview

Improves LeaderboardsController#index performance by reducing Ruby-side work when calculating “untracked entries” from the Hackatime/WakaTime heartbeats dataset.

Changes:

• Replaced distinct.pluck(...).count { ... } with a DB-side where.not(...).distinct.count(:user_id) query for counting untracked users.

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ids comes from @leaderboard.entries.pluck(:user_id) (local users.id, bigint), but Hackatime::Heartbeat.user_id appears to be the user's Slack UID (see User#hackatime_heartbeats uses primary_key: :slack_uid). Using where.not(user_id: ids) is likely a type/semantic mismatch and can raise a Postgres error (text vs bigint) or filter incorrectly. Consider converting ids to Slack UIDs first (e.g., User.where(id: ids).pluck(:slack_uid)) and filtering on those, or otherwise ensure both sides use the same identifier.

Suggested change

	Hackatime::Heartbeat.where(time: r).where.not(user_id: ids).distinct.count(:user_id)
	# `ids` are local users.id (integers), but Hackatime::Heartbeat.user_id stores Slack UIDs.
	# Convert the user IDs to Slack UIDs before filtering.
	slack_uids = User.where(id: ids).pluck(:slack_uid)
	scope = Hackatime::Heartbeat.where(time: r)
	scope = scope.where.not(user_id: slack_uids) if slack_uids.present?
	scope.distinct.count(:user_id)