WarpEngine 0.5.0: a catalog that can say a title is not yours

A desktop client reading /api/software had no way to learn that a title costs
money. There was nothing in the response to say so, no way to sign in, and no
way to be told "you do not own this" — so a store with paid titles could only
hand the client a 403 at download time and let it guess why.

The fix belongs here rather than in the client. A client serves more than one
store, so anything it knows about a particular one has to arrive from that
store's own API; a rule compiled into the client is a rule that breaks every
other catalog it reads. Three seams, each following the storage adapter's
shape — documented contract, default that is byte for byte the old behaviour,
one config key to replace it:

- **access policy** — visible_software_scope / access_for / authorize_download.
  Every catalog entry now carries an `access` block (gated, entitled, price,
  purchaseUrl, webUrl) and both /api/download and /file/* ask before serving.
  The vocabulary is deliberately generic: a word from one host's domain would
  make every client that reads it specific to that host.
- **client sign-in** — the device authorization grant (RFC 8628), over the
  host's own user model. The approval page stays the host's, because approving
  needs a session and HTML. Tokens are ApplicationTokens with a `catalog`
  scope, so publishing and reading stay separable.
- **service descriptor** — GET /api/service says what this deployment is and
  whether it has a sign-in at all, which is how a client stops guessing.

With no policy and no subject class configured — every deployment today — the
API is unchanged: /api/auth/* answers 404, /api/service reports auth: null, and
the 187 pre-existing examples pass untouched.

A policy that raises is treated as a refusal, not permission. An artifact
served because the gatekeeper crashed is the one failure mode this must not
have, so a broken policy empties the catalog and denies the download.

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

app/controllers/concerns/warp_engine/subject_authentication.rb

@@ -0,0 +1,51 @@
+module WarpEngine
+  # Who the caller is, on the read-only side of the API.
+  #
+  # Distinct from UpdateAuthentication, which guards publishing: that one asks "may this
+  # pipeline write to the catalog", this one asks "whose library am I looking at". The
+  # answer is allowed to be nobody — an anonymous caller is a normal, supported caller,
+  # and a catalog with no policy configured never needs one.
+  #
+  # The credential is a bearer token, because that is what a client can carry: it has no
+  # cookie jar and no browser session.
+  module SubjectAuthentication
+    extend ActiveSupport::Concern
+
+    private
+
+    # The ApplicationToken behind the request, or nil.
+    def current_access_token
+      return @current_access_token if defined?(@current_access_token)
+
+      @current_access_token = resolve_access_token
+    end
+
+    # Whoever that token belongs to — the host's own object. nil when the request is
+    # anonymous, when the token is unknown, or when no subject class is configured.
+    def current_subject
+      current_access_token&.owner
+    end
+
+    def resolve_access_token
+      return nil unless WarpEngine.identity_configured?
+
+      token = bearer_token
+      return nil if token.blank?
+
+      record = WarpEngine::ApplicationToken.authenticate(
+        token, required_scope: WarpEngine::ApplicationToken::CATALOG_SCOPE
+      )
+      return nil if record.nil?
+
+      record.touch_last_used!
+      record
+    end
+
+    def bearer_token
+      header = request.headers["Authorization"].to_s
+      return nil unless header.start_with?("Bearer ")
+
+      header.delete_prefix("Bearer ").strip.presence
+    end
+  end
+end