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/schema.prisma

@@ -151,6 +151,11 @@ model statements {
   account_holder_name  String?
   exchange_rate_to_aud Decimal?       @db.Decimal(10, 6)
   paperless_doc_id     Int?           @unique
+  interest_rate        Decimal?       @db.Decimal(6, 3)
+  scheduled_repayment  Decimal?       @db.Decimal(12, 2)
+  repayment_frequency  String?
+  redraw_available     Decimal?       @db.Decimal(12, 2)
+  loan_term_months     Int?
   transactions         transactions[]
 }
 
@@ -172,6 +177,8 @@ model transactions {
   amount_aud              Decimal?      @db.Decimal(12, 2)
   owner_id                Int?
   reconciled_with_id      Int?
+  principal_amount        Decimal?      @db.Decimal(12, 2)
+  interest_amount         Decimal?      @db.Decimal(12, 2)
   statement               statements?   @relation(fields: [statement_id], references: [id], onDelete: Cascade)
   reconciled_with         transactions?  @relation("reconciled", fields: [reconciled_with_id], references: [id], onDelete: SetNull)
   reconciled_by           transactions[] @relation("reconciled")