chore: optimize Dockerfile by adding cache mount for pnpm installation to improve build performance

Dockerfile

@@ -50,7 +50,8 @@ COPY --from=builder /app/prisma.config.ts ./prisma.config.ts
 
 # Install prisma CLI for migrations + better-sqlite3 (compile native module)
 ENV DATABASE_URL="file:/app/data/prod.db"
-RUN pnpm add prisma @prisma/client @prisma/adapter-better-sqlite3 better-sqlite3 dotenv && \
+RUN --mount=type=cache,id=pnpm,target=/pnpm/store \
+    pnpm add prisma @prisma/client @prisma/adapter-better-sqlite3 better-sqlite3 dotenv && \
     pnpm prisma generate
 
 # Copy entrypoint script