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/contracts/versionSet.js

@@ -0,0 +1,9 @@
+import { z } from 'zod';
+export const versionSetSchema = z.object({
+    datasetId: z.string().min(1).max(128),
+    dataHash: z.string().min(1).max(128),
+    modelVersion: z.string().min(1).max(128),
+    configVersion: z.string().min(1).max(128),
+    codeSha: z.string().min(1).max(128),
+    contractVersion: z.string().min(1).max(64)
+});