feat: failed job trend chart

[eclectic-coding]

Closes #55

Summary

• Adds failure_sparkline to DashboardStats — 12 hourly buckets of failed execution counts via a single pluck(:created_at) query, same Ruby-side bucketing pattern as the throughput and depth sparklines
• New "Failures — Last 12 Hours" card on the dashboard with red bars (sqd-sparkline__bar--failure), header total count, and empty state; positioned after the Queue Depth chart
• 3 new dashboard specs; full suite at 301 examples, 0 failures, 100% coverage

Test plan

• Visit /jobs — "Failures — Last 12 Hours" card appears between Queue Depth and the quick-links section
• With no recent failures — empty state reads "No failures in the last 12 hours"
• With failed jobs — red bars render proportionally; tooltip shows hour label and count

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