perf(dashboard): optimize user growth metrics with SQL aggregation

[MJ-RWA]

Summary

Replaced the in-memory user growth aggregation with a database-level aggregation to improve dashboard performance and reduce memory usage.

Changes Made

• Replaced full-table user reads with a GROUP BY date_trunc('month', createdAt) aggregate query using TypeORM QueryBuilder.
• Computed monthly signup counts directly in PostgreSQL.
• Replaced users.length with userRepository.count() for total user count.
• Calculated cumulative totalUsers from the aggregated monthly results while preserving the existing response format.
• Added an index on users.createdAt (if not already present) to improve aggregation performance.
• Added/updated tests to verify:
  • monthly metrics are generated using aggregation queries,
  • no full-table user read occurs,
  • the response remains unchanged,
  • the optimized query path is exercised.

Why

The previous implementation loaded every user record into memory before grouping them by month, causing memory usage to grow with the size of the users table. This change delegates aggregation to PostgreSQL, making memory usage independent of user count and improving dashboard scalability.

Result

• ✅ Database computes monthly signup metrics.
• ✅ No full-table user materialization.
• ✅ Lower memory usage during dashboard requests.
• ✅ Response format remains unchanged.
• ✅ users.createdAt is indexed for better query performance.

Closes: #1011

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