test: add sentry test page for debugging error infra

[johnshift]

Summary

• Adds /sentry-test page with buttons to trigger errors through all paths (client reporter, unhandled error, unhandled rejection, server-side captureException)
• Adds /api/sentry-test route that calls Sentry.captureException + flush() directly
• Used to verify Sentry DSN, Slack integration, and end-to-end error pipeline

Test plan

• Deploy to staging, visit /sentry-test, click each button
• Verify errors appear in Sentry dashboard (jobstash/webapp)
• Verify Slack alerts fire