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.

src/app/api/analytics/monthly/route.ts

@@ -6,6 +6,8 @@ import {
   STATEMENTS_JOIN,
   EFFECTIVE_CATEGORY,
   EXCLUDE_NON_SPEND,
+  SPEND_ROWS,
+  SPEND_BASE,
 } from "@/lib/analytics-sql";
 
 export async function GET(req: NextRequest) {
@@ -34,9 +36,9 @@ export async function GET(req: NextRequest) {
       ${EFFECTIVE_CATEGORY} as category,
       SUM(
         CASE
-          WHEN ts.share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * ts.share_percent / 100
-          WHEN o.my_share_percent IS NOT NULL THEN COALESCE(t.amount_aud, t.amount) * o.my_share_percent / 100
-          ELSE COALESCE(t.amount_aud, t.amount)
+          WHEN ts.share_percent IS NOT NULL THEN (${SPEND_BASE}) * ts.share_percent / 100
+          WHEN o.my_share_percent IS NOT NULL THEN (${SPEND_BASE}) * o.my_share_percent / 100
+          ELSE (${SPEND_BASE})
         END
       )::numeric(12,2) as total_spent,
       COUNT(*)::int as transaction_count
@@ -45,7 +47,7 @@ export async function GET(req: NextRequest) {
     LEFT JOIN transaction_splits ts ON ts.transaction_id = t.id AND ts.participant_id = $1
     ${STATEMENTS_JOIN}
     WHERE ${OWNER_SCOPE} = $1
-      AND t.transaction_type IN ('debit', 'fee', 'interest')
+      AND ${SPEND_ROWS}
       AND ${EXCLUDE_NON_SPEND}
       AND t.transaction_date >= $2
       AND t.transaction_date < $3