Table-qualify amount/expires_at scopes so they compose with joins

[rameerez]

Problem

credits / debits (and not_expired / expired) use bare column predicates:

scope :credits, -> { where("amount > 0") }
scope :debits,  -> { where("amount < 0") }

A bare amount is ambiguous the moment a host joins another table that also has an amount column — and every wallet has an amount on both wallets_transactions and wallets_transfers. So a perfectly natural query like:

# "which of my transactions belong to a reversal transfer?"
wallet.transactions.joins(:transfer).merge(Wallets::Transfer.where(category: :reversal)).credits

raises PG::AmbiguousColumn (SQLite and MySQL raise the equivalent — ambiguous column name: amount).

We hit this in production (a real-money carpooling app): computing "lifetime earned, net of reversals" needs exactly that join, and we had to work around it host-side with a two-step id resolution instead of a clean .joins(:transfer).credits.

Fix

Route the four column-referencing raw-SQL scopes through arel_table[:col], so the generated SQL is always table-qualified ("wallets_transactions"."amount" > 0). Bonus: arel_table derives from the resolved (overridable) table_name, so the embedding/subclassing story qualifies correctly for free — no hardcoded table string.

Test

New regression in transaction_test.rb:

• creates a transfer between two wallets (so both legs + a wallets_transfers.amount exist),
• runs wallet.transactions.joins(:transfer).debits/.credits — which raised before the fix (verified: reverting the scopes reproduces SQLite3::SQLException: ambiguous column name: amount),
• asserts each returns the correct leg,
• asserts the generated SQL is table-qualified (DB-agnostic).

Full suite green (93 runs, 346 assertions).