feat: add cost attribution dashboard

[dtsong]

Summary

• New /costs page with cost breakdown by time, repo, and model
• Time range selector (7d/30d/90d)
• Charts: daily cost trend, cost per review, by repo (bar), by model (pie)
• Outliers table identifying expensive reviews

Changes

• dashboard/src/app/costs/page.tsx — costs page with data aggregation
• dashboard/src/components/CostChart.tsx — reusable chart components
• dashboard/src/lib/supabase.ts — CostByRepo, CostByModel interfaces
• dashboard/src/app/page.tsx — "Costs" nav link

Test plan

• TypeScript types match existing schema
• Follows existing dashboard patterns
• Python test suite unaffected

Closes #9

🤖 Generated with Claude Code

[github-actions]

AI Code Review

Summary

The PR adds a comprehensive costs dashboard with data visualization and analysis features. The code is generally well-structured but has several potential issues with error handling, type safety, and edge cases.

Confidence: 0.00 (low)
This PR may require human review

Strengths

• Well-structured component architecture with reusable chart components
• Comprehensive data visualization covering multiple cost dimensions
• Good use of TypeScript interfaces for type safety
• Responsive design with proper grid layouts
• Includes outlier detection for identifying expensive reviews
• Follows existing codebase patterns and conventions

Issues Found

Major (3)

dashboard/src/app/costs/page.tsx:31 - logic

Database query lacks error handling - if the query fails, data will be null but the code assumes it succeeded
Suggestion: Add proper error handling for the database query to handle failures gracefully

dashboard/src/app/costs/page.tsx:85 - logic

Outliers calculation has a logical flaw - when avgCost is 0, the filter condition avgCost > 0 prevents any outliers from being found, but reviews could still have non-zero costs
Suggestion: Fix the outliers logic to handle the case where average cost is 0 but individual reviews have costs

dashboard/src/components/CostChart.tsx:119 - logic

Pie chart label extraction assumes model names follow a specific format with hyphens, which could break with different model naming conventions
Suggestion: Make the model name extraction more robust and handle various naming formats

Minor (3)

dashboard/src/app/costs/page.tsx:49 - logic

Date extraction using split('T')[0] is fragile and could break with different date formats
Suggestion: Use proper date parsing to extract the date part more reliably

dashboard/src/lib/supabase.ts:45 - testing

New TypeScript interfaces CostByRepo and CostByModel are defined but not used in the current implementation
Suggestion: Either use these interfaces in the code or remove them if they're not needed yet

dashboard/src/app/costs/page.tsx:47 - performance

Multiple reduce operations iterate over the same reviews array, which could be inefficient for large datasets
Suggestion: Consider combining the reduce operations into a single pass for better performance

Suggestion (2)

dashboard/src/app/costs/page.tsx:43 - style

Inline styles make the component harder to maintain and test
Suggestion: Consider extracting styles to CSS modules or styled components for better maintainability

dashboard/src/app/costs/page.tsx:152 - logic

Hard-coded limit of 8 repositories in the chart could hide important cost data
Suggestion: Consider making this configurable or showing all repositories with costs above a threshold

Concerns

• Lack of error handling for database operations could cause the page to crash
• Multiple data aggregation operations could be performance bottlenecks with large datasets
• Hardcoded assumptions about data formats (dates, model names) could break with different data

Questions

• Are the CostByRepo and CostByModel interfaces intended for future use, or should they be implemented now?
• Should there be loading states for individual chart components during data processing?
• Is there a maximum expected number of reviews this page should handle efficiently?
• Should the outliers threshold (2x average) be configurable?

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_{Model: claude-sonnet-4-20250514 | Cost: $0.0403 |
Tokens: 5833 in / 1518 out}