OTWO-7638 Add performance findings for admin dashboard

[kaushak-ai]

This document outlines the performance issues identified in the admin dashboard, including N+1 queries, expensive window-function job lookups, and missing indexes. Recommendations for optimization are provided, along with a caching strategy and validation plan to measure improvements.

[Copilot]

Pull request overview

This PR updates admin dashboard stats queries and chart generation to reduce query cost (avoiding join-induced duplicates, reducing grouping work, and improving index usage) while adding defensive handling for missing/zero baselines in percentage calculations.

Changes:

• Replaces join+distinct.size patterns with EXISTS-based counting for “projects with enlistments” stats (and caches the results).
• Updates dashboard helper percentage methods to avoid divide-by-zero and normalizes cached values to integers.
• Refactors admin project chart monthly gap filling to precompute monthly totals and avoid repeated per-month queries; also adjusts date filtering to avoid DATE(created_at) casts.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
lib/tasks/admin_project_stats.rake	Changes how “active projects (with enlistments)” is counted and cached.
app/helpers/dashboard_helper.rb	Updates cached counts and percentage calculations (including divide-by-zero handling).
app/decorators/oh_admin/project_chart.rb	Refactors monthly chart total calculation and simplifies grouped queries.
app/decorators/oh_admin/account_chart.rb	Replaces DATE(created_at) predicate with range-friendly timestamp comparison.

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