Account key for search records

Add an account key field to improve search performance. This field
allows us to filter the records by account directly in the fulltext
index, so we only need to examine rows belonging to the relevant account.

db/migrate/20251121092508_add_account_key_to_search_records.rb

@@ -0,0 +1,37 @@
+class AddAccountKeyToSearchRecords < ActiveRecord::Migration[8.2]
+  def up
+    16.times do |shard_id|
+      table_name = "search_records_#{shard_id}"
+
+      # Add account_key column
+      add_column table_name, :account_key, :string, null: false, default: ""
+
+      # Backfill existing records with account_key
+      execute <<-SQL
+        UPDATE #{table_name}
+        SET account_key = CONCAT('account', account_id)
+      SQL
+
+      # Create new fulltext index including account_key
+      add_index table_name, [:account_key, :content, :title], type: :fulltext
+
+      # Drop existing fulltext index
+      remove_index table_name, column: [:content, :title], type: :fulltext
+    end
+  end
+
+  def down
+    16.times do |shard_id|
+      table_name = "search_records_#{shard_id}"
+
+      # Drop new fulltext index
+      remove_index table_name, column: [:account_key, :content, :title], type: :fulltext
+
+      # Restore original fulltext index
+      add_index table_name, [:content, :title], type: :fulltext
+
+      # Remove account_key column
+      remove_column table_name, :account_key
+    end
+  end
+end