[ANALYTICS-001] Add product analytics events and dashboard

[chev0004]

Outline

Summary

Adds product analytics: a privacy-aware, first-party event-tracking system with a user opt-out and an admin-only metrics dashboard.

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Changes & Enhancements

• Type of Change:
  • Feature
  • Bugfix
  • Refactor
  • Chore
• Changes:
  • (analytics):
    • Central event catalog and a privacy-aware server trackEvent that never throws and always records locale.
    • Metadata sanitizer that drops everything outside a non-sensitive allowlist, so no private profile content is sent.
    • Client beacon helper and /api/analytics ingest endpoint guarded by a client-event allowlist.
  • (instrumentation):
    • Signup, login, onboarding start and complete, profile save, bump, save-to-favorites, discovery view, profile view, and username copy.
    • Anonymous and authenticated events are both supported.
  • (settings):
    • Product analytics opt-out toggle, enforced server-side before any event is written.
  • (dashboard):
    • Admin-gated /[lang]/analytics with an activation funnel, daily volume, event volume, and recent events.
  • (db):
    • analytics_events table and migration, plus a product_analytics settings column.
  • (i18n):
    • English and Japanese strings for the dashboard and the new setting.

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Page / App Changes

Affected Next.js Routes (Local):

• /[lang]/analytics (new admin dashboard)
• /[lang]/settings (new Product analytics privacy toggle)
• /[lang], /[lang]/u/[id], /[lang]/onboarding (emit view events, no visual change)
• /api/analytics (new client event ingest endpoint)

Before (Screenshot)

After (Screenshot)

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Component / Storybook Changes

Affected Storybook Components (Local):

• Features/Analytics/AnalyticsDashboard > Default
• Features/Analytics/AnalyticsDashboard > Empty
• Features/Settings/SettingsPage > PrivacyTab

Before (Screenshot)

After (Screenshot)

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Not Doing

• Events for features that do not exist yet (reports, blocks, intro requests, premium upgrades) are defined in the catalog but not emitted; they will be wired up when those features land.

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Other (Remarks)

Run bun run db:migrate after merge (adds analytics_events and the product_analytics column). The dashboard is gated by a new POLYCORD_ADMIN_USER_IDS env var (comma-separated Discord IDs), documented in .env.example.

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Checklists

• Code is well-documented (comments, JSDoc, etc.).
• Existing comments were updated as needed.
• No out-of-scope changes are included.
• Write TODO comments where future work is required.
• Removed unnecessary debug code (e.g., console.log, debugger).
• Self-reviewed and tested locally.