Search for operator words instead of parsing them

The search box keeps only word characters, but FTS5 still reads AND, OR,
NOT and NEAR in the query as operators, so searching for "AND" or for
"salt AND" raised "fts5: syntax error" and answered with a 500. Quoting
each word makes FTS5 look for it as text, which is how it already treats
the same words in lower case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NuTtwJKhb77Dv7EQqv2C3C

app/models/message/searchable.rb

@@ -6,7 +6,15 @@ module Message::Searchable
     after_update_commit  :update_in_index
     after_destroy_commit :remove_from_index
 
-    scope :search, ->(query) { joins("join message_search_index idx on messages.id = idx.rowid").where("idx.body match ?", query).ordered }
+    scope :search, ->(query) { joins("join message_search_index idx on messages.id = idx.rowid").where("idx.body match ?", match_terms(query)).ordered }
+  end
+
+  class_methods do
+    # Quotes each word, so that FTS5 searches for AND, OR, NOT and NEAR rather than
+    # parsing them as operators, which fails on a query like "AND" or "salt AND".
+    def match_terms(query)
+      query.split.map { |word| %("#{word.gsub('"', '""')}") }.join(" ")
+    end
   end
 
   private