fix(06-03): make prisma postinstall conditional for Docker multi-stage build

In Docker deps stage only package.json files are copied (no schema),
causing prisma generate to fail. Builder stage already runs explicit
prisma generate with full source.

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

apps/api/package.json

@@ -7,7 +7,7 @@
     "start": "node dist/main.js",
     "start:dev": "nest start --watch",
     "type-check": "tsc --noEmit",
-    "postinstall": "prisma generate"
+    "postinstall": "test -f prisma/schema.prisma && prisma generate || true"
   },
   "dependencies": {
     "@microsoft/microsoft-graph-client": "3.0.7",