Add sub_provider field to User model and UI

[Copilot]

Summary
Fixes two issues and adds a database migration to resolve schema inconsistency:

Zatladder Controller: When no Amber users exist, fallback to sofia_account users instead of crashing:

users = User.in_amber.any? ? User.in_amber : User.sofia_account
@users_spendings = users.calculate_spendings(from:, to:)
OTP Authentication: Accept 1-period drift to handle clock skew between client and server:

sofia_account.authenticate_otp(params[:verification_code], drift: 1)
Database Migration: Adds order_total column back to orders table if missing, resolving schema.rb inconsistency.

Other information
Migration is idempotent using column_exists? check to safely run on environments where the column may already exist.

[coderabbitai]

Important

Review skipped

Bot user detected.

To trigger a single review, invoke the @coderabbitai review command.

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}