Fix notification preferences and retention

[arjunkomath]

Summary

• apply alert-category toggles consistently to email and in-app notifications
• clarify notification settings copy across both channels
• delete notifications read more than 30 days ago with a daily Inngest cron
• report the cleanup count from PostgreSQL without returning deleted rows

Verification

• pnpm exec vitest run tests/notifications.test.ts tests/inngest-route.test.ts
• ./node_modules/.bin/tsc --noEmit
• Biome check on touched files

Basecamp: https://app.basecamp.com/5890597/buckets/47439993/todos/10158024611

[github-actions]

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_{Reviewed by React Doctor for commit d594c5c.}