chore: migrate Prisma config from package.json to prisma.config.ts

Fixes deprecation warning:
"The configuration property 'package.json#prisma' is deprecated and
will be removed in Prisma 7."

Changes:
- Created apps/api/prisma.config.ts with seed configuration
- Removed deprecated "prisma" field from apps/api/package.json
- Uses defineConfig from "prisma/config" per Prisma 6+ standards

Migration verified with successful prisma generate.

Refs https://pris.ly/prisma-config

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/package.json

@@ -23,9 +23,6 @@
     "prisma:seed": "prisma db seed",
     "prisma:reset": "prisma migrate reset"
   },
-  "prisma": {
-    "seed": "tsx prisma/seed.ts"
-  },
   "dependencies": {
     "@mosaic/shared": "workspace:*",
     "@nestjs/common": "^11.1.12",

apps/api/prisma.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from "prisma/config";
+
+export default defineConfig({
+  migrations: {
+    seed: "tsx prisma/seed.ts",
+  },
+});