Optimize heartbeat duration calculations and indexes

[seer-by-sentry]

Fixes HACKATIME-19H. The issue was that: Complex window functions without optimal index support cause sequential scans and sorts, leading to slow queries.

• Add a new database index on heartbeats for (user_id, language, time) to optimize window functions partitioned by language.
• Refactor daily_durations_for_users to improve index usage by simplifying the inner window function's PARTITION BY to user_id and moving day grouping to the outer aggregation.
• Enhance total_duration_for_scope to dynamically include user_id in window function PARTITION BY clauses when user_id is present in the query's WHERE clause, leveraging composite indexes.

This fix was generated by Seer in Sentry, triggered by mahadkalam1234@gmail.com. 👁️ Run ID: 9956850

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