fix(identity-service): add DATABASE_URL for Prisma generate in Dockerfile

Fix build failure caused by missing DATABASE_URL environment variable
during prisma:generate step. Changed from npm run prisma:generate to
direct npx prisma generate with dummy DATABASE_URL.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/services/identity-service/Dockerfile

@@ -8,9 +8,13 @@ COPY prisma ./prisma/
 
 RUN npm ci
 
+# Generate Prisma client (dummy DATABASE_URL for build time only)
+RUN DATABASE_URL="postgresql://user:pass@localhost:5432/db" npx prisma generate
+
+# Copy source code
 COPY . .
 
-RUN npm run prisma:generate
+# Build TypeScript
 RUN npm run build
 
 # Production stage