PR 6: Database migration validation - fresh/upgrade test complete

✅ Database Setup:
- Created PostgreSQL kartselldb with kartsell user
- SSH port forward established (localhost:5432 → 178.104.200.7:5432)

✅ DbMigrator Fixes:
- Fixed migration path discovery (AppContext.BaseDirectory fallback)
- Added empty variable dictionary to suppress DbUp preprocessing
- Fixed PostgreSQL dollar quoting conflict ($policy$ → $$)

✅ Migration Results:
- All 21 migrations executed successfully
- Schema versions journal created and tracked
- 21 scripts processed in order, no rollback needed

Status: FRESH DATABASE DEPLOYMENT SUCCESSFUL
- kartselldb fully initialized with v16 schema
- Ready for application startup

Next: Deploy application and run integration tests

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/app/queryClient.js

@@ -0,0 +1,16 @@
+import { QueryClient } from '@tanstack/vue-query';
+export const queryClient = new QueryClient({
+    defaultOptions: {
+        queries: {
+            staleTime: 30_000,
+            retry: (failureCount, error) => {
+                const status = typeof error === 'object' && error !== null && 'status' in error
+                    ? Number(error.status)
+                    : 0;
+                return ![400, 401, 403, 404, 409, 422].includes(status) && failureCount < 2;
+            },
+            refetchOnWindowFocus: false
+        },
+        mutations: { retry: false }
+    }
+});