feat: add analytics and usage metrics to dashboard (review counts, finding trends, adoption)

[charannyk06]

Priority: P3 — Low

Problem

The dashboard has placeholder usage sections but no real analytics or metrics. There is no way to see:

• How many reviews have been run over time
• What types of findings are most common (security vs bugs vs architecture)
• Which files/directories have the most issues
• Review completion rate and timing

Solution

1. Collect metrics at review time

In chainreview-api, when syncing review results:

• Count findings by severity and agent type
• Record review duration
• Record file count analyzed

2. Dashboard analytics page

Add charts to the dashboard (Recharts is already a dependency):

Review Activity
[Bar chart: reviews per day last 30 days]

Finding Distribution
[Pie chart: Security 40% | Bugs 35% | Architecture 25%]

Severity Breakdown
[Stacked bar: Critical/High/Medium/Low per week]

Top Issue Files
[List: src/auth/login.ts - 8 findings | src/api/routes.ts - 5 findings]

3. Trend line

Show improvement over time: "↓ 23% fewer high severity findings this month"

4. Export data

Allow downloading analytics as CSV for external analysis.

Note on Privacy

All analytics are aggregated — no code content is sent. File paths may optionally be excluded for sensitive repos (setting: chainreview.analytics.excludeFilePaths).

Acceptance Criteria

• Review metrics stored in Supabase on each sync
• Dashboard shows review activity chart
• Finding distribution by type and severity
• Top files by finding count
• Export to CSV