chore: remove Sentry integration

Remove @sentry/nextjs and all related files since it was never actively used.
- Delete sentry.server.config.ts, sentry.edge.config.ts
- Delete sentry-example-page and sentry-example-api routes
- Clean up instrumentation.ts, global-error.tsx, middleware.ts
- Remove Sentry env vars from env.example and docs
- Update CLAUDE.md, copilot-instructions.md, PRODUCTION_READINESS.md

Middleware bundle reduced from 86KB to 34.8KB (-51KB).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

env.example

@@ -48,6 +48,4 @@ PRISMA_AUTO_BASELINE=false
 # SKIP_PRISMA_MIGRATE=true
 
 # Monitoring (optional)
-NEXT_PUBLIC_SENTRY_DSN=your-sentry-dsn
-SENTRY_AUTH_TOKEN=your-sentry-auth-token
 LOG_LEVEL=info