feat: maximize Sentry free plan coverage for frontend and backend

- ErrorBoundary: use Sentry.ErrorBoundary with crash feedback dialog
- RouteError: capture route errors in Sentry (skip chunk load errors)
- User context: set Sentry user on login (frontend + backend)
- Backend: enable profiling (profiles_sample_rate)
- Frontend: add feedback integration, lower replay rate to conserve quota
- Add temporary verification message for production validation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/common/RouteError.tsx

@@ -1,5 +1,6 @@
 import { useEffect } from 'react'
 import { useRouteError, isRouteErrorResponse, useNavigate } from 'react-router-dom'
+import * as Sentry from '@sentry/react'
 import { Button } from '@/components/ui/Button'
 
 function isChunkLoadError(error: unknown): boolean {
@@ -19,6 +20,13 @@ export function RouteError() {
   const error = useRouteError()
   const navigate = useNavigate()
 
+  // Report route errors to Sentry (skip chunk load errors — those are deploy artifacts)
+  useEffect(() => {
+    if (error && !isChunkLoadError(error)) {
+      Sentry.captureException(error)
+    }
+  }, [error])
+
   // Auto-reload once on chunk load failures (stale deploy)
   useEffect(() => {
     if (isChunkLoadError(error)) {