fix(server): run assets.deploy as a mix release step

Without this, 'mix release' produced a tarball that had app.css/app.js
(so LiveView worked) but was missing cache_manifest.json and the digested
asset paths. Phoenix served the bare files OK, but the client-side
LiveView bootstrap timing was fragile: if a form was submitted before
the LiveSocket attached, the browser fell back to a native HTML GET,
producing bug-report URLs like /admin/hosts?host%5Bname%5D=repl.

Define a project releases/0 with a pre-assemble step that runs
assets.deploy, so minified + digested assets are baked into every
release tarball.

Also gitignore digested priv/static artifacts so dev-time byproducts
don't pollute commits.

server/mix.exs

@@ -9,10 +9,30 @@ defmodule Server.MixProject do
       elixirc_paths: elixirc_paths(Mix.env()),
       start_permanent: Mix.env() == :prod,
       aliases: aliases(),
-      deps: deps()
+      deps: deps(),
+      releases: releases()
     ]
   end
 
+  defp releases do
+    [
+      server: [
+        # Ensure JS/CSS are built, minified and digested before assembling the
+        # release. Without this step, priv/static/assets/cache_manifest.json is
+        # missing from the tarball — the browser still gets app.js (LiveView
+        # works) but phx-track-static cache-busting is broken and any form
+        # submitted during a dead-link window falls through to a native HTML
+        # GET.
+        steps: [&build_assets/1, :assemble]
+      ]
+    ]
+  end
+
+  defp build_assets(release) do
+    Mix.Task.run("assets.deploy")
+    release
+  end
+
   # Configuration for the OTP application.
   #
   # Type `mix help compile.app` for more information.