Implement Prisma database layer

[Spbd1]

Motivation

• Introduce a first-class, type-safe PostgreSQL data layer using Prisma to persist Parcel Society domain objects and relationships.
• Model the full experimental domain (servers, seasons, rounds, players, parcels, decisions, contracts, events, treasury, audit, checks, exports) to support gameplay, admin, and export workflows.
• Provide an idempotent seed for development demos and document migration and client generation steps for local development.

Description

• Add a complete Prisma schema at packages/db/prisma/schema.prisma defining enums, models, relations, indexes, and uniqueness constraints for the requested core entities and types.
• Add a reusable Prisma client singleton to packages/db/src/client.ts with development hot-reload protection and re-export Prisma types/enums from packages/db/src/index.ts.
• Replace the placeholder seed with packages/db/scripts/seed.ts, which idempotently creates one super-admin user/admin profile and four demo servers covering the 2x2 treatment design (low/high inequality × stable/uncertain institutions).
• Update README.md with migration, client generation, and seeding instructions (pnpm db:migrate, pnpm db:generate, pnpm seed) and a short Docker example for starting PostgreSQL.

Testing

• Ran git diff --check to validate whitespace and basic diff checks and found no issues.
• Attempted pnpm --filter @parcel-society/db db:generate to generate the Prisma client, but it could not run because the prisma binary and node_modules are not available in this environment (client generation is environment-dependent).
• Attempted pnpm install to install dev dependencies locally, but it failed due to npm registry access returning 403 Forbidden, so automated schema migration/generation and package-level tests could not be executed here.

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Codex Task