Events page: analytics panels (volume chart, top events, platform breakdown) (#186)

[unjica]

Summary

• Adds GET /api/events/analytics (registered before /events/:id) returning filtered volume series, top event names with share %, and platform breakdown (Web / iOS / Android / Other).
• Introduces events-analytics.ts with bucketing aligned to overview/errors patterns and Vitest coverage for merge helpers and platform normalization.
• Wires EventsAnalyticsPanels on the Events page — line chart, top-events bars, and platform donut — fetched in parallel with summary KPIs (shared metricsUntil anchor).

Test plan

• pnpm lint && pnpm test && pnpm build
• /review-bugbot against develop — no findings
• Open Events page with default filters — panels render above table
• Change app/environment/platform/date filters — panels and KPIs stay aligned
• All-time range uses rolling 7-day metrics window with stable metricsUntil

Closes #186

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