Errors page summary KPIs, in-range counts, and release filter (#181)

[unjica]

Summary

• Adds GET /api/errors/summary with KPI metrics (total errors, affected users, error rate, unique/resolved groups) and prior-period comparison.
• Scopes error list Count to occurrences_in_range (aligned with summary window when no explicit date range is set).
• Adds Release filter to the errors list API and dashboard toolbar; filter options now union event and error-occurrence releases.
• Renders a summary KPI row on /dashboard/errors.

Fixes #181

Test plan

• pnpm lint && pnpm test && pnpm build
• Open /dashboard/errors — KPI row shows with prior-period deltas
• Change date range — summary and Count column update together
• Filter by release — list and summary both narrow
• With no date range — summary shows "Last 7 days" and counts match that window

Made with Cursor

[cursor]

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