feat: PostgreSQL support en Vercel CI/CD deployment

- Prisma schema: sqlite → postgresql provider met directUrl
- prisma.config.ts: directUrl toegevoegd
- seed.ts: dynamisch SQLite of PostgreSQL adapter op basis van DATABASE_URL
- ci.yml: preview deploy op PR, productie deploy op push naar main

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

prisma/schema.prisma

@@ -3,7 +3,9 @@ generator client {
 }
 
 datasource db {
-  provider = "sqlite"
+  provider  = "postgresql"
+  url       = env("DATABASE_URL")
+  directUrl = env("DIRECT_URL")
 }
 
 enum Role {