Add settings model (#54)

* Adds a basic settings model

* Added more settings methods; Hooked up initial settings runner to app boot

* Update onboarding flow to use settings model instead of session data

priv/repo/migrations/20240306173305_create_settings.exs

@@ -0,0 +1,15 @@
+defmodule Pinchflat.Repo.Migrations.CreateSettings do
+  use Ecto.Migration
+
+  def change do
+    create table(:settings) do
+      add :name, :string, null: false
+      add :value, :string, null: false
+      add :datatype, :string, null: false
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create unique_index(:settings, [:name])
+  end
+end