server: database schema, migrations and seed

Define Prisma schema (Player, City, Good, InventoryItem, Transaction,
Mission, WorldEvent), initial migration SQL, and seed script to
populate cities and goods with base game data.

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

server/prisma/migrations/migration_lock.toml

@@ -0,0 +1,3 @@
+# Please do not edit this file manually
+# It should be added in your version-control system (e.g., Git)
+provider = "postgresql"