chore(tooling): switch to fnm + pnpm, add DEPLOYMENT.md

Pin Node 24 via .node-version/.nvmrc and pnpm 9.15.0 via
package.json#packageManager. Regenerate lockfile as pnpm-lock.yaml.
Rewrite README setup + scripts table around pnpm, and add a
production deployment guide covering systemd, nginx, upgrades,
rollback, and backups.

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

package.json

@@ -3,12 +3,13 @@
   "version": "0.1.0",
   "private": true,
   "type": "module",
+  "packageManager": "pnpm@9.15.0",
   "scripts": {
     "dev": "vite dev",
     "build": "vite build",
     "preview": "vite preview",
     "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --threshold warning",
-    "validate": "npm run check && npm run build",
+    "validate": "pnpm run check && pnpm run build",
     "db:generate": "drizzle-kit generate",
     "db:migrate": "drizzle-kit migrate",
     "db:push": "drizzle-kit push",