Update series_name for alt styles to be null, with a fallback string

I considered this at first, but decided to keep it simple until it
turned out to matter. Oops, it already matters, lol!

I want the item search code to be able to easily tell if the series
name is real or a placeholder, so we can decide whether to build the
filter text in `fits:$series-$color-$species` form or
`fits:alt-style-$id` form.

So in this change, we keep it that `AltStyle#series_name` returns the
placeholder string if none is set, but callers can explicitly ask
whether it's a real series name or not. Will use this in our next
change!

app/models/alt_style.rb

@@ -17,6 +17,20 @@ class AltStyle < ApplicationRecord
                    species_human_name: species.human_name)
   end
 
+  # If the series_name hasn't yet been set manually by support staff, show the
+  # string "<New?>" instead. But it won't be searchable by that string—that is,
+  # `fits:<New?>-faerie-draik` intentionally will not work, and the canonical
+  # filter name will be `fits:alt-style-IDNUMBER`, instead.
+  def series_name
+    self[:series_name] || "<New?>"
+  end
+
+  # You can use this to check whether `series_name` is returning the actual
+  # value or its placeholder value.
+  def has_real_series_name?
+    self[:series_name].present?
+  end
+
   def adjective_name
     "#{series_name} #{color.human_name}"
   end

db/migrate/20240227233743_change_default_for_alt_styles_series_name.rb

@@ -0,0 +1,6 @@
+class ChangeDefaultForAltStylesSeriesName < ActiveRecord::Migration[7.1]
+  def change
+    change_column_null :alt_styles, :series_name, true
+    change_column_default :alt_styles, :series_name, from: "<New?>", to: nil
+  end
+end

db/schema.rb

@@ -10,14 +10,14 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2024_02_27_231815) do
+ActiveRecord::Schema[7.1].define(version: 2024_02_27_233743) do
   create_table "alt_styles", charset: "utf8mb4", collation: "utf8mb4_unicode_ci", force: :cascade do |t|
     t.integer "species_id", null: false
     t.integer "color_id", null: false
     t.integer "body_id", null: false
     t.datetime "created_at", precision: nil, null: false
     t.datetime "updated_at", precision: nil, null: false
-    t.string "series_name", default: "<New?>", null: false
+    t.string "series_name"
     t.index ["color_id"], name: "index_alt_styles_on_color_id"
     t.index ["species_id"], name: "index_alt_styles_on_species_id"
   end