fix: pin all sharp platform binaries so npm ci works on any host

The previous attempts (--include=optional, then a separate npm install
fallback) failed because npm ci runs sharp's install script DURING
installation — and that script crashes ("Please add node-gyp to your
dependencies") before the next Dockerfile step gets to run.

Real fix: pin every sharp platform binary as an optionalDependency in
package.json. npm now records URL+hash for all of them in the lock
file regardless of which OS generated the lock. On any build host,
npm ci picks the matching binary via the os/cpu/libc filters in those
packages and silently skips the rest.

Pinned binaries (sharp 0.34.5):
- @img/sharp-linuxmusl-x64   (Alpine x64 — our VPS)
- @img/sharp-linuxmusl-arm64 (Alpine arm64)
- @img/sharp-linux-x64       (glibc x64)
- @img/sharp-linux-arm64     (glibc arm64)
- @img/sharp-darwin-arm64    (Apple Silicon dev)
- @img/sharp-darwin-x64      (Intel Mac dev)

Side benefit: simplifies the Dockerfile. Drops the secondary
`npm install --no-save --cpu=x64 --os=linux --libc=musl sharp` step
and the vips-dev system package (no source compilation needed when
the prebuilt binary is guaranteed present). The runner stage still
needs `vips` runtime, that stays.

package-lock.json

@@ -51,6 +51,14 @@
         "prisma": "^7.5.0",
         "tailwindcss": "^4",
         "typescript": "^5"
+      },
+      "optionalDependencies": {
+        "@img/sharp-darwin-arm64": "0.34.5",
+        "@img/sharp-darwin-x64": "0.34.5",
+        "@img/sharp-linux-arm64": "0.34.5",
+        "@img/sharp-linux-x64": "0.34.5",
+        "@img/sharp-linuxmusl-arm64": "0.34.5",
+        "@img/sharp-linuxmusl-x64": "0.34.5"
       }
     },
     "node_modules/@ai-sdk/gateway": {