Post-V1: migrate Prisma seed config off package.json#prisma

[NexaStack-Software]

After v1.0.0, Prisma still warns that package.json#prisma is deprecated and will be removed in Prisma 7.\n\nScope:\n- Move the seed configuration to a supported Prisma config file.\n- Keep all existing generators working: Kysely, Prisma Client, JSON types, Zod types.\n- Verify npm run prisma:generate, npm run build, and the release gate subset still pass.\n\nContext:\nAn earlier quick migration attempt before v1.0.0 was intentionally not shipped because generator output changed unexpectedly. Treat this as a careful post-release maintenance task, not a hotfix.

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[NexaStack-Software]

Implemented in 78bb551. Added packages/prisma/prisma.config.ts with schema and migrations.seed, removed package.json#prisma, and verified via the V1 release gate. prisma:generate, build, migrate deploy, and the release E2E subset all pass; the package.json#prisma deprecation warning is gone and all configured generators still run.