The backend image starts: approve the native builds, compile sharp against the system libvips

Built and run for the first time (until now the dockerized path had only been
checked with `docker compose config`): the image built, and the container
exited at start with `Cannot find module '../build/Release/sharp-linux-x64.node'`.
pnpm 10 skips dependency build scripts unless they are approved, so sharp,
bcrypt and sodium-native were installed without their binaries. A local
node_modules can carry binaries from an earlier manual build, which is how it
stayed invisible. `pnpm.onlyBuiltDependencies` now approves the three.

Approved, sharp's installer then downloaded its prebuilt libvips and binary
from GitHub, and that download timed out on every in-build attempt (3 of 3),
failing the build, while the same download from a plain container was
instantaneous. With libvips-dev installed, sharp detects a system libvips and
compiles itself instead; npm_config_nodedir points node-gyp at the headers the
image already ships. Verified with GitHub and nodejs.org blackholed: the build
passes, sharp loads against libvips 8.14.1, bcrypt falls back to compiling.

Cost: a larger image; a multi-stage build keeping only libvips42 at runtime
would drop several hundred MB.

backend/package.json

@@ -34,5 +34,12 @@
   "engines": {
     "node": ">=22.12"
   },
+  "pnpm": {
+    "onlyBuiltDependencies": [
+      "bcrypt",
+      "sharp",
+      "sodium-native"
+    ]
+  },
   "packageManager": "pnpm@10.26.0+sha512.3b3f6c725ebe712506c0ab1ad4133cf86b1f4b687effce62a9b38b4d72e3954242e643190fc51fa1642949c735f403debd44f5cb0edd657abe63a8b6a7e1e402"
 }