Phase 5: ship the catalog ActiveAdmin resources from the engine

- the 7 catalog admin files (softwares, releases, external_links,
  platform_links, images, files page, downloads) move to the engine's
  app/admin; the host ActiveAdmin instance loads them via
  ActiveAdmin.application.load_paths (single admin, URLs unchanged)
- engine initializers: Zeitwerk ignore for app/admin (production eager load)
  and load_paths + watchable_dirs registration, guarded by defined?(ActiveAdmin)
  so the admin-less dummy app boots
- images admin reads image owners from WarpEngine.config.image_owners; the
  host registers the Member owner in config/initializers/warp_engine.rb;
  the interim ImageUsage registry is gone
- fix: SoftwareImage.distinct.pluck clashed with its order(:position)
  default scope on MySQL (unscope(:order)) — introduced in phase 0, caught
  by the first authenticated /admin/images smoke test
- files page: download links use the public /file/ URL instead of the raw
  container path; the picker's stored path comes from config

Verified: both suites green, zeitwerk:check (production) clean, JSON
baselines intact, authenticated admin smoke test on every page incl.
the 3-level nested software form and Files picker mode.

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

app/admin/platform_links.rb

@@ -0,0 +1,52 @@
+ActiveAdmin.register WarpEngine::PlatformLink, as: "Platform Link" do
+  permit_params :name, :url, :platform, :position
+
+  menu priority: 5, label: "🔗 Platform Links"
+
+  config.sort_order = "platform_asc"
+
+  scope :all, default: true
+  scope("TIC-80")     { |s| s.where(platform: "tic80") }
+  scope("Ebitengine") { |s| s.where(platform: "ebitengine") }
+  scope("LOVE")       { |s| s.where(platform: "love") }
+  scope("C64")        { |s| s.where(platform: "c64") }
+  scope("Godot")      { |s| s.where(platform: "godot") }
+  scope("Bevy")       { |s| s.where(platform: "bevy") }
+  scope("Phaser")     { |s| s.where(platform: "phaser") }
+
+  index do
+    selectable_column
+    id_column
+    column :platform
+    column :name
+    column(:url) { |pl| link_to pl.url, pl.url, target: "_blank" }
+    column :position
+    column :created_at
+    actions
+  end
+
+  filter :platform, as: :select, collection: %w[tic80 ebitengine love c64 godot bevy phaser]
+  filter :name
+
+  form do |f|
+    f.inputs do
+      f.input :platform, as: :select, collection: %w[tic80 ebitengine love c64 godot bevy phaser]
+      f.input :name
+      f.input :url
+      f.input :position, as: :number, input_html: { min: 0 }
+    end
+    f.actions
+  end
+
+  show do
+    attributes_table do
+      row :id
+      row :platform
+      row :name
+      row(:url) { |pl| link_to pl.url, pl.url, target: "_blank" }
+      row :position
+      row :created_at
+      row :updated_at
+    end
+  end
+end