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