Harden vectorization task against per-package failures

A single failing package used to kill its whole worker thread silently
because only Vectorizer::Error was rescued per package. Now every
StandardError is caught and logged with class and backtrace, and the
embedding write is narrowed to update_column(:embedding) so the task
only ever issues a minimal UPDATE - it cannot touch any other column
or fire model callbacks. Progress stats no longer serialize database
writes through the mutex.

app/models/package.rb

@@ -56,13 +56,12 @@ class Package < ApplicationRecord
 
   # Compute and store the embedding vector for this package by asking
   # the external vector service. Raises Vectorizer::Error on failure.
-  # Returns true if the vector was saved.
+  # Returns true if a vector was saved.
   def update_embedding!
     text = embedding_text
     return false if text.blank?
 
-    self.embedding = Vectorizer.embed(text)
-    save!
+    update_column(:embedding, Vectorizer.embed(text))
   end
 
   # Return a relation of packages whose stored embeddings are closest to

lib/tasks/vectorize_packages.rake

@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+# Long rake task bodies are normal; the block-length cop is meant for
+# application code.
+# rubocop:disable Metrics/BlockLength
 namespace :debshots do
   desc 'Compute embedding vectors for all packages based on their description ' \
        'and long description (missing ones only unless FORCE=1). ' \
@@ -34,17 +37,20 @@ namespace :debshots do
     stats = { done: 0, failed: 0 }
 
     process_package = lambda do |package|
-      stats_mutex.synchronize do
       begin
         raise ArgumentError, 'no description text' if package.embedding_text.blank?
 
         package.update_embedding!
-          stats[:done] += 1
-        rescue Vectorizer::Error, ArgumentError => e
-          stats[:failed] += 1
-          logger.error "Failed for package #{package.name}: #{e.message}"
+        stats_mutex.synchronize { stats[:done] += 1 }
+      rescue StandardError => e
+        # A single broken package must never take down a whole worker
+        # thread - log enough detail to diagnose it and move on.
+        stats_mutex.synchronize { stats[:failed] += 1 }
+        logger.error "Failed for package #{package.name}: #{e.class}: #{e.message}"
+        logger.error e.backtrace.first(3).join("\n") if e.backtrace
       end
 
+      stats_mutex.synchronize do
         processed = stats[:done] + stats[:failed]
         return unless (processed % 100).zero?