fix(fleet): resolve getSenchoVersion crash in Docker containers (#396)

* fix(fleet): resolve getSenchoVersion crash in Docker containers

Replace the runtime __dirname walk (which fails in Docker because the
root package.json is absent from the final image) with a build-time
version injection. A prebuild script reads the root package.json and
generates src/generated/version.ts with the version baked in as a
constant. The compiled output carries the correct version regardless
of the container's filesystem layout.

Retains a filesystem walk fallback for dev environments where the
prebuild hook may not have run.

* fix(fleet): skip postinstall version generation when script is absent

The Docker prod-deps stage copies only package.json (no source files)
before running npm ci --omit=dev. The postinstall hook now checks for
the script file before executing, so it no-ops in stages where
scripts/generate-version.js has not been copied.

backend/package.json

@@ -4,8 +4,11 @@
   "description": "",
   "main": "index.js",
   "scripts": {
+    "postinstall": "test -f scripts/generate-version.js && node scripts/generate-version.js || true",
+    "prebuild": "node scripts/generate-version.js",
     "build": "tsc",
     "start": "node dist/index.js",
+    "predev": "node scripts/generate-version.js",
     "dev": "nodemon --watch src --ext ts,json --exec ts-node src/index.ts",
     "test": "vitest run",
     "lint": "eslint src"