fix: restore admin log context filters

[stevene1919]

Summary

• Fix the Admin System Event Log Context Filter to consume the API response's contexts field.
• Keep pagination state on the requested page; the API returns pages but not page.
• Add typed frontend contracts for log entries and the logs response, removing the any response type.
• Update the changelog.

Testing

• pnpm --filter pricestalker-frontend test -- --run
• pnpm --filter pricestalker-frontend run build
• pnpm --filter pricestalker-backend test -- --run
• pnpm --filter pricestalker-backend run build
• pnpm run lint
• git diff --check

The broader event-log JSON schema and UX redesign from the issue discussion is intentionally left for a separate follow-up.

Closes #78