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/shared/api/client.js

@@ -0,0 +1,18 @@
+import axios from 'axios';
+import { ApiProblem } from './problem';
+export const api = axios.create({ baseURL: '/api', timeout: 15_000 });
+api.interceptors.request.use(config => {
+    const user = import.meta.env.VITE_DEV_AUTH_USER;
+    const role = import.meta.env.VITE_DEV_AUTH_ROLE;
+    if (import.meta.env.DEV && user && role) {
+        config.headers['X-KArtSell-User'] = user;
+        config.headers['X-KArtSell-Role'] = role;
+    }
+    return config;
+});
+api.interceptors.response.use(response => response, error => {
+    const data = error.response?.data;
+    if (data?.status)
+        throw new ApiProblem(data);
+    throw error;
+});