fix(maestro): pin pnpm via packageManager so corepack doesn't grab v11

The Dockerfile's `web-builder` stage (NAP-218) uses Node 20 + corepack.
Without a `packageManager` field, corepack defaulted to pnpm@latest,
which is now 11.3.0 — that release requires Node ≥22.13 and imports
`node:sqlite`, so it dies on Node 20 with
`ERR_UNKNOWN_BUILTIN_MODULE: node:sqlite`. Dokploy builds were failing
in the `pnpm install` step.

Pin pnpm@9.15.4 (latest 9.x, fully Node-20 compatible). No code change
to the web bundle itself.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/package.json

@@ -3,6 +3,7 @@
   "private": true,
   "version": "0.1.0",
   "type": "module",
+  "packageManager": "pnpm@9.15.4",
   "scripts": {
     "dev": "vite",
     "build": "tsc -b && vite build",