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/store/authStore.ts

@@ -1,5 +1,6 @@
 import { create } from 'zustand'
 import { persist } from 'zustand/middleware'
+import * as Sentry from '@sentry/react'
 import type { User, Token, UserCreate, UserLogin, Account, SubscriptionDetails } from '@/types'
 import { authApi } from '@/api/auth'
 import { apiClient } from '@/api/client'
@@ -81,6 +82,7 @@ export const useAuthStore = create<AuthState>()(
           localStorage.removeItem('access_token')
           localStorage.removeItem('refresh_token')
           clearCachedQuota()
+          Sentry.setUser(null)
           set({ user: null, token: null, account: null, subscription: null, isAuthenticated: false, error: null })
         }
       },
@@ -104,6 +106,9 @@ export const useAuthStore = create<AuthState>()(
             throw reason
           }
 
+          // Set Sentry user context for error attribution
+          Sentry.setUser({ id: user.id, email: user.email })
+
           set({ user, account, subscription, isLoading: false })
         } catch (error: unknown) {
           const message = error instanceof Error ? error.message : 'Failed to fetch user'