Fix prod docker build: native modules and frontend devDeps

Per-package onlyBuiltDependencies so pnpm 10 actually compiles native
modules (isolated-vm, argon2, sqlite3, sharp) — root workspace config
isn't visible when compose mounts a single package into /app.

Frontend container now installs with devDeps (so tsc/vite exist),
prunes after build, and gains a `start` script that serves the build
via vite preview on 0.0.0.0:3000.

Drop obsolete compose `version` key.

backend/package.json

@@ -18,5 +18,13 @@
   "devDependencies": {
     "nodemon": "3.1.14",
     "uuid": "14.0.0"
+  },
+  "pnpm": {
+    "onlyBuiltDependencies": [
+      "argon2",
+      "isolated-vm",
+      "sharp",
+      "sqlite3"
+    ]
   }
 }