Fix production boot crash: install libffi8 in Railway runtime image (#1869)

* Fix production boot crash: install libffi8 in Railway runtime image

web and worker have been crashing on boot since 2026-08-19 with
LoadError: libffi.so.8: cannot open shared object file, from the ffi
gem's native extension (required transitively via sassc -> sassc-rails).
Railpack doesn't auto-detect libffi as a runtime dependency for Ruby's
ffi gem, and something changed on Railway's builder side that stopped
carrying libffi.so.8 into the deploy image. Explicitly declaring it in
railpack.json's deploy.aptPackages restores it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Document why libffi8 is needed in railpack.json

Explains the ffi -> sassc -> sassc-rails -> rails_admin chain and the
Railpack v0.37.0 Debian 13 migration that dropped libffi.so.8 from the
runtime image, so this doesn't look like an arbitrary apt package pin.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: Yanchek99

railpack.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "https://schema.railpack.com",
+  "deploy": {
+    // The ffi gem (native ext, required via sassc -> sassc-rails, which
+    // rails_admin's Sprockets asset pipeline needs) links against
+    // libffi.so.8 at boot. Railpack doesn't auto-detect libffi as a
+    // runtime dependency, and Railpack v0.37.0's move to Debian 13
+    // (Trixie) base images stopped carrying it into the deploy image,
+    // crashing web and worker on boot. "..." extends Railpack's
+    // auto-detected apt package list rather than replacing it.
+    "aptPackages": ["...", "libffi8"]
+  }
+}