Implement official Prisma solution for client generation

Following Prisma's official documentation for deployment caching issues:
https://www.prisma.io/docs/orm/more/help-and-troubleshooting/vercel-caching-issue

Changes:
- Add 'prisma generate' to build script (official Prisma recommendation)
- Add postinstall script for automatic client generation
- Remove custom stub generator workaround
- Keep runtime Prisma client generation in entrypoint.sh for reliability
- Add openssl to runtime container (required for Prisma engines)

This follows Prisma best practices: explicitly run prisma generate during the
build process to ensure Prisma Client is always up-to-date. The entrypoint
script regenerates the client at runtime to guarantee engine availability in
the production environment.

docker/web/entrypoint.sh

@@ -18,6 +18,16 @@ gosu nextjs sh -c '
     # Set npm cache to writable directory
     export NPM_CONFIG_CACHE=/tmp/.npm
 
+    # Generate real Prisma client at runtime (replaces build-time stub)
+    echo "Generating Prisma client..."
+    npx prisma generate || {
+        echo "Warning: Prisma generate failed, attempting with checksum ignore..."
+        PRISMA_ENGINES_CHECKSUM_IGNORE_MISSING=1 npx prisma generate || {
+            echo "Error: Failed to generate Prisma client"
+            exit 1
+        }
+    }
+
     if [ ! -f "$DB_PATH" ]; then
         echo "Database not found, initializing..."
         npx prisma db push --skip-generate