Rewrite a message's search entry only when its body is saved

Boosting and unboosting touch the message, so after_update_commit
rewrote its row in the full-text index with the same text, loading the
rich text again to rebuild it, on every boost. The index only needs a
new row when the rich text body was saved.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0142qgjggdJ2KDdGk7RF9Xm9

app/models/message/searchable.rb

@@ -3,7 +3,7 @@ module Message::Searchable
 
   included do
     after_create_commit  :create_in_index
-    after_update_commit  :update_in_index
+    after_update_commit  :update_in_index, if: :body_saved?
     after_destroy_commit :remove_from_index
 
     scope :search, ->(query) { joins("join message_search_index idx on messages.id = idx.rowid").where("idx.body match ?", match_terms(query)).ordered }
@@ -22,6 +22,11 @@ module Message::Searchable
       execute_sql_with_binds "insert into message_search_index(rowid, body) values (?, ?)", id, plain_text_body
     end
 
+    # Boosts touch their message too: rewrite the index only when the body was saved.
+    def body_saved?
+      association(:rich_text_body).target&.saved_change_to_body?
+    end
+
     def update_in_index
       execute_sql_with_binds "update message_search_index set body = ? where rowid = ?", plain_text_body, id
     end