Transaction analysis: clean spending figures

Rawabi's transactions table contains every posting: salaries, transfers, ATM withdrawals, fees, profit payments, card purchases, and the reversals of some of those purchases. Spending in this lesson means card and online purchases:

• channel IN ('pos', 'online') (point-of-sale and online purchases);
• direction = 'debit' (money out of the customer's account);
• merchant_name IS NOT NULL (online rent payments and transfers to other people are debits on the online channel too, but they have no merchant and are not purchases);
• not reversed: a reversal is a separate credit row, 1–4 days later, with reversal_of_transaction_id pointing at the original purchase.

Internal transfers between Rawabi accounts (card repayments, savings top-ups) use the transfer channel, so the channel rule already keeps them out. Counting them would double-count money that simply moved between two accounts of the same customer.