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>

middleware.ts

@@ -70,9 +70,7 @@ export function middleware(request: NextRequest) {
     pathname.startsWith("/api/") ||
     pathname === "/api" ||
     pathname.startsWith("/manage") ||
-    pathname.startsWith("/editor") ||
-    pathname === "/sentry-example-page" ||
-    pathname.startsWith("/sentry-example-page/");
+    pathname.startsWith("/editor");
 
   // Locale routing for public site pages
   const responseUrl = request.nextUrl.clone();