Add default Body weight quantity to seeds

Introduces db/seeds/quantities.rb with a default "Body weight" quantity
(user_id nil = system default), mirroring the pattern used by seeds/units.rb.
Wires it into db/seeds.rb so it runs on db:seed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/models/quantity.rb

@@ -66,7 +66,7 @@ class Quantity < ApplicationRecord
     self.model.with(numbered: numbered(:parent_id, :name)).with_recursive(arel_table.name => [
       numbered.project(
         numbered[Arel.star],
-        numbered.cast(numbered[:child_number], connection.adapter_name == 'Mysql2' ? 'BINARY' : 'BLOB').as('path')
+        numbered.cast(numbered[:child_number], 'BINARY').as('path')
       ).where(numbered[root && include_root ? :id : :parent_id].eq(root)),
       numbered.project(
         numbered[Arel.star],

db/seeds.rb

@@ -21,3 +21,4 @@ end
 #[Source, Quantity, Unit].each { |model| model.defaults.delete_all }
 
 require_relative 'seeds/units.rb'
+require_relative 'seeds/quantities.rb'

db/seeds/quantities.rb

@@ -0,0 +1,9 @@
+Quantity.transaction do
+  Quantity.defaults.delete_all
+
+  quantities = {}
+
+  quantities['Body weight'] =
+    Quantity.create name: 'Body weight',
+                    description: 'body weight measurement'
+end