feat(loans): principal/interest split so repayments stop distorting spend

A loan repayment is not an expense. A $3,000 mortgage repayment is roughly
$1,200 of principal (equity — a balance-sheet move) and $1,800 of interest (the
only part that is genuinely spend).

Migration 0014:
- transactions.principal_amount / interest_amount, populated only when the lender
  itemises the split on the repayment row
- statements.interest_rate, scheduled_repayment, repayment_frequency,
  redraw_available, loan_term_months
- normalize_repayment_frequency() + trigger, so "Fortnightly", "Bi-Weekly" and
  "Every 2 weeks" all land on 'fortnightly'

Two statement shapes are handled. Where the loan statement lists repayments and
"Interest Charged" as separate rows (the common Australian case), transaction_type
already does the work. Where a lender itemises the split on the repayment row,
that row is typed 'payment' and would be skipped entirely — losing the interest.
New SPEND_ROWS / SPEND_BASE fragments in analytics-sql.ts count such rows at
interest_amount instead of amount.

Adds the loan_interest category (+ colour, and the missing fees colour).

Verified against the live DB with a synthetic ANZ home loan statement: a $3,000
itemised repayment plus a $10 service fee moved April spend by exactly $1,810,
with the $1,200 principal excluded and still retained on the row. Test data
removed and the figure confirmed back at its original value.

prisma/migrations/0014_loan_statements/migration.sql

@@ -0,0 +1,85 @@
+-- Loan statement support.
+--
+-- Migration 0013 taught the system that a statement can be a loan; this gives
+-- loans somewhere to put the data that only loans have.
+--
+-- The core accounting problem: a loan repayment is not an expense. A $3,000
+-- mortgage repayment is roughly $1,200 of principal (a balance-sheet move that
+-- builds equity) and $1,800 of interest (the only part that is genuinely spend).
+-- Counting the whole repayment as spending overstates expenses badly.
+--
+-- Two statement shapes are handled:
+--   (a) The common Australian case — the loan statement lists repayments and
+--       "Interest Charged" as separate rows. transaction_type already carries
+--       this: 'interest' rows count as spend, 'payment' rows do not.
+--   (b) Some lenders itemise principal and interest on the repayment row itself.
+--       That's what principal_amount / interest_amount are for: when
+--       interest_amount is set, analytics count that instead of the full amount.
+--
+-- Idempotent: safe to re-run.
+
+-- Per-transaction principal/interest split (shape (b) above).
+ALTER TABLE transactions
+  ADD COLUMN IF NOT EXISTS principal_amount NUMERIC(12,2),
+  ADD COLUMN IF NOT EXISTS interest_amount  NUMERIC(12,2);
+
+COMMENT ON COLUMN transactions.principal_amount IS
+  'Principal portion of a loan repayment, when the statement itemises it. Not spend.';
+COMMENT ON COLUMN transactions.interest_amount IS
+  'Interest portion of a loan repayment, when the statement itemises it. This is the part that counts as spend.';
+
+-- Partial index: only loan repayment rows carry a split.
+CREATE INDEX IF NOT EXISTS idx_transactions_interest_amount
+  ON transactions (interest_amount)
+  WHERE interest_amount IS NOT NULL;
+
+-- Loan-level terms, read off the statement header.
+ALTER TABLE statements
+  ADD COLUMN IF NOT EXISTS interest_rate        NUMERIC(6,3),
+  ADD COLUMN IF NOT EXISTS scheduled_repayment  NUMERIC(12,2),
+  ADD COLUMN IF NOT EXISTS repayment_frequency  TEXT,
+  ADD COLUMN IF NOT EXISTS redraw_available     NUMERIC(12,2),
+  ADD COLUMN IF NOT EXISTS loan_term_months     INTEGER;
+
+COMMENT ON COLUMN statements.interest_rate IS 'Annual interest rate as a percentage, e.g. 6.140';
+COMMENT ON COLUMN statements.redraw_available IS 'Funds available to redraw (loans) — not the same as available_credit on a card.';
+
+-- Free text varies by lender ("Monthly", "Fortnightly"); normalise the common
+-- spellings rather than constraining, so an unexpected value never blocks an import.
+ALTER TABLE statements DROP CONSTRAINT IF EXISTS statements_repayment_frequency_check;
+
+CREATE OR REPLACE FUNCTION normalize_repayment_frequency(raw TEXT)
+RETURNS TEXT AS $$
+DECLARE
+  v TEXT := lower(trim(coalesce(raw, '')));
+BEGIN
+  IF v = '' THEN RETURN NULL; END IF;
+  -- Check fortnightly spellings before the bare '%week%' match below.
+  IF v LIKE '%fortnight%' OR v LIKE '%bi-week%' OR v LIKE '%biweek%'
+     OR v LIKE '%2 week%' OR v LIKE '%two week%' OR v LIKE '%14 day%'
+  THEN RETURN 'fortnightly'; END IF;
+  IF v LIKE '%month%'   THEN RETURN 'monthly'; END IF;
+  IF v LIKE '%week%'    THEN RETURN 'weekly'; END IF;
+  IF v LIKE '%quarter%' THEN RETURN 'quarterly'; END IF;
+  IF v LIKE '%annual%' OR v LIKE '%year%' THEN RETURN 'annually'; END IF;
+  RETURN v;
+END;
+$$ LANGUAGE plpgsql IMMUTABLE;
+
+CREATE OR REPLACE FUNCTION statements_normalize_loan_fields_trigger()
+RETURNS TRIGGER AS $$
+BEGIN
+  NEW.repayment_frequency := normalize_repayment_frequency(NEW.repayment_frequency);
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS trg_statements_normalize_loan_fields ON statements;
+CREATE TRIGGER trg_statements_normalize_loan_fields
+  BEFORE INSERT OR UPDATE OF repayment_frequency ON statements
+  FOR EACH ROW EXECUTE FUNCTION statements_normalize_loan_fields_trigger();
+
+UPDATE statements
+SET repayment_frequency = normalize_repayment_frequency(repayment_frequency)
+WHERE repayment_frequency IS NOT NULL
+  AND repayment_frequency IS DISTINCT FROM normalize_repayment_frequency(repayment_frequency);