feat(cash): mark how a transaction was paid, exclude cash from reconciliation

getPendingReconciliations treated every unreconciled manual transaction as
awaiting a matching statement row. Cash never appears on a statement, so a cash
entry sat in the queue indefinitely being offered matches within 3 days and 1%
on amount - and accepting one is silently destructive: reconciled manual rows
are filtered out of every query, so the cash spend disappears while the card
transaction it matched claims to be that same spend.

Migration 0016 adds transactions.payment_method (card | cash | bank_transfer |
other, NULL = unknown) with a CHECK constraint and a partial index. The notCash()
fragment excludes cash from both halves of the reconciliation query - the pending
list and the candidate match subquery, which aliases the manual row as m.

Only cash is excluded. Bank transfers do appear on a statement now that
transaction accounts are imported, and NULL means unknown, so both stay
candidates and every pre-existing row behaves exactly as before.

ATM withdrawals deliberately stay categorised as spend rather than transfers.
Treating them as transfers is only correct if every cash purchase is logged;
with partial logging it silently deletes the unlogged remainder from spend.

prisma/schema.prisma

@@ -175,6 +175,7 @@ model transactions {
   row_index               Int?
   merchant_normalized     String?
   amount_aud              Decimal?      @db.Decimal(12, 2)
+  payment_method          String?       // card | cash | bank_transfer | other; NULL = unknown (migration 0016)
   owner_id                Int?
   reconciled_with_id      Int?
   principal_amount        Decimal?      @db.Decimal(12, 2)