Block SSRF via backchannel_logout_uri

backchannel_logout_uri was validated only for scheme/HTTPS, so an admin (or a
compromised admin account) could point it at internal infrastructure — cloud
metadata (169.254.169.254), loopback, or RFC1918 hosts — and every user logout
would fire a server-side POST there.

Add PrivateAddressCheck (app/lib) and apply it as defense-in-depth:
- Application validation rejects URIs whose host is, or is a literal, internal
  address (loopback / private / link-local / 0.0.0.0 / localhost / metadata
  hostnames). Fast, DNS-free, immediate admin feedback.
- BackchannelLogoutJob re-checks at request time WITH DNS resolution and aborts
  (no retry) if the host resolves to a non-public address — covering URIs that
  predate the validation and public hostnames pointed at internal IPs.

Tests cover the address classification, the model validation, and updates an
existing test that used a localhost logout URI.

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

app/models/application.rb

@@ -56,6 +56,7 @@ class Application < ApplicationRecord
     message: "must be a valid HTTP or HTTPS URL"
   }
   validate :backchannel_logout_uri_must_be_https_in_production, if: -> { backchannel_logout_uri.present? }
+  validate :backchannel_logout_uri_not_internal, if: -> { backchannel_logout_uri.present? }
 
   # Icon validation using ActiveStorage validators
   validate :icon_validation
@@ -390,4 +391,17 @@ class Application < ApplicationRecord
       # Let the format validator handle invalid URIs
     end
   end
+
+  # SSRF guard: the backchannel logout URI is dialled server-side on every user
+  # logout, so it must not target internal infrastructure (loopback, private
+  # ranges, or the link-local cloud metadata endpoint). This is the fast,
+  # config-time check; BackchannelLogoutJob re-checks with DNS resolution.
+  def backchannel_logout_uri_not_internal
+    uri = URI.parse(backchannel_logout_uri)
+    if uri.host.present? && PrivateAddressCheck.internal_host?(uri.host)
+      errors.add(:backchannel_logout_uri, "must not point to a private, loopback, or link-local address")
+    end
+  rescue URI::InvalidURIError
+    # Let the format validator handle invalid URIs
+  end
 end