chore(prisma): point root postinstall to BFF schema

`@prisma/client`'s postinstall hook runs `prisma generate` from the
workspace root, where it doesn't find a schema in the default
locations and warns. The hook then exits without generating the
typed client — fine today (the schema has no models yet, no code
calls into the client at runtime), but a trap waiting for the first
real model to be added.

Declare `package.json#prisma.schema` so the postinstall hook locates
the schema at apps/portal-bff/prisma/schema.prisma and generates the
typed client unconditionally on every `pnpm install`.

Note: Prisma 6.19 emits a deprecation warning ("migrate to
prisma.config.ts"). Migrating that format will be folded into the
future Prisma 7 upgrade (which itself depends on nestjs-prisma
catching up to Prisma 7 and will get its own ADR).

package.json

@@ -32,6 +32,9 @@
       "unrs-resolver"
     ]
   },
+  "prisma": {
+    "schema": "apps/portal-bff/prisma/schema.prisma"
+  },
   "devDependencies": {
     "@analogjs/vite-plugin-angular": "~2.1.2",
     "@analogjs/vitest-angular": "~2.1.2",