Extract detached-blob guard into its own file

Separate the attachment existence check from the broadcast suppression
override so each file has a single responsibility. The guard now lives
in ActiveStorageAnalyzeJobSkipDetached with its own documentation
explaining the upload-then-delete race condition.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/rails_ext/active_storage_analyze_job_skip_detached.rb

@@ -0,0 +1,17 @@
+# Skip analysis for blobs whose attachments have already been destroyed.
+#
+# When an upload is deleted before AnalyzeJob runs, PurgeOnLastAttachment has
+# already destroyed the attachment row and enqueued PurgeJob for the blob's S3
+# object. Analyzing at this point hits a missing key. Same check as
+# PurgeOnLastAttachment#purge_blob_if_last.
+module ActiveStorageAnalyzeJobSkipDetached
+  def perform(blob)
+    return unless blob.attachments.exists?
+
+    super
+  end
+end
+
+ActiveSupport.on_load :active_storage_blob do
+  ActiveStorage::AnalyzeJob.prepend ActiveStorageAnalyzeJobSkipDetached
+end

lib/rails_ext/active_storage_analyze_job_suppress_broadcasts.rb

@@ -4,8 +4,6 @@
 # there is currently a bug https://github.com/rails/rails/issues/55144
 module ActiveStorageAnalyzeJobSuppressBroadcasts
   def perform(blob)
-    return unless blob.attachments.exists?
-
     Board.suppressing_turbo_broadcasts do
       Card.suppressing_turbo_broadcasts do
         super