Add index on readouts(user_id, taken_at)

MeasurementsController#index orders by taken_at desc; without an index
this scan grows linearly with the readout count. The composite index
on (user_id, taken_at) covers both the implicit user_id filter from
the association scope and the ORDER BY clause.

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

db/schema.rb

@@ -39,6 +39,7 @@ ActiveRecord::Schema[7.2].define(version: 2026_04_03_000000) do
     t.index ["quantity_id"], name: "index_readouts_on_quantity_id"
     t.index ["unit_id"], name: "index_readouts_on_unit_id"
     t.index ["user_id"], name: "index_readouts_on_user_id"
+    t.index ["user_id", "taken_at"], name: "index_readouts_on_user_id_and_taken_at"
   end
 
   create_table "units", charset: "utf8mb4", collation: "utf8mb4_0900_as_ci", force: :cascade do |t|