chore(netlify): add netlify.toml for automatic admin frontend deploys

Configures Netlify to build apps/admin on every push to main:
- Installs pnpm, generates Prisma client, builds shared-types, then admin
- Publishes apps/admin/dist with SPA redirect fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

netlify.toml

@@ -0,0 +1,12 @@
+[build]
+  base    = "."
+  command = "npm install -g pnpm && pnpm install --frozen-lockfile && pnpm --filter @repo/database generate && pnpm --filter @repo/shared-types build && pnpm --filter admin build"
+  publish = "apps/admin/dist"
+
+[build.environment]
+  NODE_VERSION = "20"
+
+[[redirects]]
+  from   = "/*"
+  to     = "/index.html"
+  status = 200