One home per rule: error bodies, mime types, asset paths, ordering

Four small duplications, each of which had already drifted or was one edit away
from it.

**The JSON error body.** The host's `ApiController` was a line-by-line copy of
the engine's four `rescue_from` blocks plus `resolve_mime`. It is a
`WarpEngine::ApiErrorRendering` concern now, included by both, so "what a
failure looks like on the wire" is decided once.

**Mime resolution.** The same three lines lived in the engine's API controller
and the host's. `WarpEngine::Storage.mime_for` owns it — next to the adapter
that hands out the files.

**Finding the asset behind a path.** `DownloadService` and `FileService` each
escaped `%` and `_` by hand and ran their own `LIKE` — the second one without
the exact-match attempt the first one had. `ReleaseAsset.for_relative_path` is
the one lookup: the absolute path first, then a suffix match anchored at `/`
rather than the old `%path%`, which could match a different file whose name
merely contained this one. Two now-unreachable helpers
(`DownloadService.base_path`, `FileService.base_path`) go with it.

**Ordering.** `PlatformLink` and `SoftwareImage` ordered inside their
`default_scope`, which leaks into every association and aggregate — the proof
was already in the tree: `admin/images.rb` had to write
`SoftwareImage.unscope(:order).distinct.pluck(:image_id)`, because MySQL will
not order a DISTINCT by a column it does not select. Ordering is a scope you
ask for now (`ordered`), the two places that need it ask (`for_platform`, the
`software_images` association, so the API's image order is unchanged), and the
`unscope` is gone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/models/warp_engine/release_asset.rb

@@ -13,6 +13,11 @@ module WarpEngine
 
     default_scope { where(deleted_at: nil) }
 
+    def self.for_relative_path(relative)
+      absolute = File.join(WarpEngine.config.file_container_path, relative.to_s)
+      find_by(path: absolute) || where("path LIKE ?", "%/#{sanitize_sql_like(relative.to_s)}").first
+    end
+
     def self.ransackable_attributes(auth_object = nil)
       %w[created_at deleted_at id kind path release_id updated_at]
     end

app/models/warp_engine/software_image.rb

@@ -13,7 +13,7 @@ module WarpEngine
 
     before_save :unset_other_defaults, if: -> { is_default? && is_default_changed? }
 
-    default_scope { order(:position) }
+    scope :ordered, -> { order(:position) }
 
     def self.ransackable_attributes(auth_object = nil)
       %w[created_at id image_id is_default position software_id updated_at]