Bring simple signup flow from the fizzy-saas gem

We skip the QB code and we fill external account ids automatically on creation with a sequence

See:
https://github.com/basecamp/fizzy-saas/pull/7

db/migrate/20251127000001_create_account_external_id_sequences.rb

@@ -1,6 +1,6 @@
 class CreateAccountExternalIdSequences < ActiveRecord::Migration[8.0]
   def change
-    create_table :account_external_id_sequences do |t|
+    create_table :account_external_id_sequences, id: :uuid do |t|
       t.bigint :value, null: false, default: 0
 
       t.index :value, unique: true

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.2].define(version: 2025_11_25_130010) do
+ActiveRecord::Schema[8.2].define(version: 2025_11_27_000001) do
   create_table "accesses", id: :uuid, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.datetime "accessed_at"
     t.uuid "account_id", null: false
@@ -25,6 +25,11 @@ ActiveRecord::Schema[8.2].define(version: 2025_11_25_130010) do
     t.index ["user_id"], name: "index_accesses_on_user_id"
   end
 
+  create_table "account_external_id_sequences", id: :uuid, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "value", default: 0, null: false
+    t.index ["value"], name: "index_account_external_id_sequences_on_value", unique: true
+  end
+
   create_table "account_join_codes", id: :uuid, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.uuid "account_id", null: false
     t.string "code", null: false

db/schema_sqlite.rb

@@ -25,7 +25,7 @@ ActiveRecord::Schema[8.2].define(version: 2025_11_27_000001) do
     t.index ["user_id"], name: "index_accesses_on_user_id"
   end
 
-  create_table "account_external_id_sequences", force: :cascade do |t|
+  create_table "account_external_id_sequences", id: :uuid, force: :cascade do |t|
     t.bigint "value", default: 0, null: false
     t.index ["value"], name: "index_account_external_id_sequences_on_value", unique: true
   end