feat: Add PR open time distribution histogram [issue: #167]

[abdullahxyz85]

Summary [Issue: #167 ]

Adds a histogram showing how long PRs stay open before being merged, broken into time buckets to reveal code review velocity patterns.

What's Changed

• New horizontal bar chart in PRMetrics component showing PR open time distribution
• 4 time buckets: < 1h, 1-24h, 1-7d, > 7d
• Uses recharts BarChart with responsive sizing
• Interactive tooltips showing exact PR count per bucket
• Respects existing 30-day range filter (ready for future range selection)
• All styling uses CSS variables for theme consistency

Implementation Details

• Modified /api/metrics/prs/route.ts to calculate and return time distribution data
• Updated PRMetrics.tsx to render histogram below existing stats
• Added TypeScript interfaces for proper type safety
• Maintains backward compatibility with existing metrics

Visual Preview

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Acceptance Criteria Met [issue: #167 ]

• ✅ New chart in PR Analytics widget
• ✅ 4 time buckets with intuitive labels
• ✅ Horizontal bar chart using recharts
• ✅ Data from existing /api/metrics/prs endpoint
• ✅ Respects 30-day range
• ✅ Uses CSS variables for colors
• ✅ Tooltip shows exact count on hover

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[abdullahxyz85]

Hi @Priyanshu-byte-coder ,
I hope u are doing well.
I completed the issue #167 , please review and merge it!
Please add the label gssoc:approved, and level: advanced or any other label that's required for gssoc contribution. or assign section add me.