feat(integrity): balance assertions, category constraint, refund netting

Four related fixes to spend correctness.

Balance assertions. Nothing checked that a statement's transactions add up to
its closing balance. getStatements now computes opening + movement - closing
and the statements page flags any statement that does not reconcile. Sign
depends on what the balance means: on a credit card or loan it is what you owe,
so spending increases it; on a transaction or offset account it is what you
hold. 11 statements currently fail, $4,177 unexplained - including two adjacent
ANZ statements off by exactly +/-$230.38, a transaction filed against the wrong
one.

Category normalisation (migration 0015). Categories arrive from Gemini (which
writes straight to Postgres from N8N), CSV import and manual edits, so the rule
belongs in the database - same reasoning as normalize_statement_type in 0013.
Adds normalize_category(), triggers on transactions and transaction_overrides,
and CHECK constraints. Backfilled 122 rows: 19 'payment' ($42,569) to transfers,
14 'refund' recovered to the merchant's usual category, and title-case duplicates
folded into their canonical spelling - 'Shopping' and 'shopping' had been
counted as separate categories by every GROUP BY.

Refund netting. Monthly analytics counted only debits, so a refund was counted
nowhere: excluded from spend by type, and not income by category. A $2,888.92
Expedia purchase refunded in full eight days later still read as $2,888.92 of
spend. SPEND_SIGNED and NET_SPEND_ROWS bring refunds in as negatives; 'income'
joins the excluded categories so incoming money cannot leak in as negative
spend. $36,773 across 93 rows now nets correctly.

Drill-down share. The insights drill-down used my_share_percent ?? 100, which
ignored transaction_splits entirely and repeated the bug fixed in ab00f8c one
layer up. getTransactions now returns my_share_pct and my_amount resolved
server-side, and the table shows gross alongside your share.

prisma/migrations/0015_category_normalization/migration.sql

@@ -0,0 +1,137 @@
+-- Canonical category vocabulary, enforced at the database.
+--
+-- Categories arrive from three places: the Gemini extraction (which writes
+-- straight to Postgres from N8N, bypassing the app entirely), CSV import, and
+-- manual edits. Only the DB sits under all three, so that is where the rule has
+-- to live -- the same reasoning as normalize_statement_type() in 0013.
+--
+-- What had leaked in without it:
+--   payment    19 rows  $42,569.42   transaction_type written into the category
+--   refund     15 rows   $8,558.99   ditto
+--   Shopping   13 rows   $2,917.44   title-case duplicates of real categories,
+--   Dining     10 rows     $127.30   which every GROUP BY counted separately
+--   (NULL)      3 rows   $1,135.05
+
+CREATE OR REPLACE FUNCTION normalize_category(raw TEXT)
+RETURNS TEXT AS $$
+DECLARE
+  v TEXT;
+BEGIN
+  IF raw IS NULL OR btrim(raw) = '' THEN RETURN 'other'; END IF;
+
+  -- Title-case and spaced variants collapse onto the canonical spelling:
+  -- 'Home Goods' -> 'home_goods', 'Shopping' -> 'shopping'.
+  v := replace(replace(lower(btrim(raw)), ' ', '_'), '-', '_');
+
+  IF v IN ('groceries','dining','transport','fuel','shopping','utilities',
+           'entertainment','travel','health','insurance','subscriptions',
+           'cash_advance','government','education','rent','home_goods',
+           'home_maintenance','transfers','income','investment','loan_interest',
+           'personal_care','pets','gifts','charity','fees','other')
+  THEN RETURN v; END IF;
+
+  -- transaction_type leaking into the category field. A payment is money moving
+  -- to a card or account, which is exactly what 'transfers' means here.
+  IF v IN ('payment','payments','transfer') THEN RETURN 'transfers'; END IF;
+
+  -- Bare 'interest' is credit-card interest; loan interest is categorised
+  -- loan_interest by the extraction prompt and matches the list above.
+  IF v IN ('fee','bank_fees','interest') THEN RETURN 'fees'; END IF;
+
+  -- Singular/plural and common synonyms.
+  IF v IN ('grocery','supermarket')        THEN RETURN 'groceries'; END IF;
+  IF v IN ('subscription')                 THEN RETURN 'subscriptions'; END IF;
+  IF v IN ('utility','bills')              THEN RETURN 'utilities'; END IF;
+  IF v IN ('gift')                         THEN RETURN 'gifts'; END IF;
+  IF v IN ('pet')                          THEN RETURN 'pets'; END IF;
+  IF v IN ('restaurant','restaurants','food','takeaway') THEN RETURN 'dining'; END IF;
+  IF v IN ('petrol','gas','gasoline')      THEN RETURN 'fuel'; END IF;
+  IF v IN ('medical','pharmacy')           THEN RETURN 'health'; END IF;
+  IF v IN ('donation','donations')         THEN RETURN 'charity'; END IF;
+  IF v IN ('salary','wages')               THEN RETURN 'income'; END IF;
+  IF v IN ('investments','savings')        THEN RETURN 'investment'; END IF;
+  IF v IN ('housing','mortgage')           THEN RETURN 'rent'; END IF;
+
+  -- A 'refund' is not a category -- it says nothing about what was bought. The
+  -- backfill below recovers the real category from the merchant where it can;
+  -- anything still unknown lands in 'other' rather than inventing a category.
+  RETURN 'other';
+END;
+$$ LANGUAGE plpgsql IMMUTABLE;
+
+
+-- Recover categories for rows typed as refunds before collapsing them to
+-- 'other': use the most common category that merchant has on ordinary spend.
+UPDATE transactions t
+SET category = m.mode_category
+FROM (
+  SELECT merchant_normalized,
+         MODE() WITHIN GROUP (ORDER BY category) AS mode_category
+  FROM transactions
+  WHERE merchant_normalized IS NOT NULL
+    AND category IS NOT NULL
+    AND category NOT IN ('payment','refund','fee','interest')
+    AND transaction_type IN ('debit','fee','interest')
+  GROUP BY merchant_normalized
+) m
+WHERE t.merchant_normalized = m.merchant_normalized
+  AND t.category = 'refund'
+  AND m.mode_category IS NOT NULL;
+
+
+-- Backfill everything else through the function.
+UPDATE transactions
+SET category = normalize_category(category)
+WHERE category IS NULL OR category <> normalize_category(category);
+
+UPDATE transaction_overrides
+SET category_override = normalize_category(category_override)
+WHERE category_override IS NOT NULL
+  AND category_override <> normalize_category(category_override);
+
+
+CREATE OR REPLACE FUNCTION transactions_normalize_category()
+RETURNS TRIGGER AS $$
+BEGIN
+  NEW.category := normalize_category(NEW.category);
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS trg_transactions_normalize_category ON transactions;
+CREATE TRIGGER trg_transactions_normalize_category
+  BEFORE INSERT OR UPDATE OF category ON transactions
+  FOR EACH ROW EXECUTE FUNCTION transactions_normalize_category();
+
+-- Overrides keep NULL meaning "no override"; only a set value is normalised.
+CREATE OR REPLACE FUNCTION overrides_normalize_category()
+RETURNS TRIGGER AS $$
+BEGIN
+  IF NEW.category_override IS NOT NULL THEN
+    NEW.category_override := normalize_category(NEW.category_override);
+  END IF;
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS trg_overrides_normalize_category ON transaction_overrides;
+CREATE TRIGGER trg_overrides_normalize_category
+  BEFORE INSERT OR UPDATE OF category_override ON transaction_overrides
+  FOR EACH ROW EXECUTE FUNCTION overrides_normalize_category();
+
+
+ALTER TABLE transactions DROP CONSTRAINT IF EXISTS transactions_category_check;
+ALTER TABLE transactions ADD CONSTRAINT transactions_category_check
+  CHECK (category IS NULL OR category IN (
+    'groceries','dining','transport','fuel','shopping','utilities','entertainment',
+    'travel','health','insurance','subscriptions','cash_advance','government',
+    'education','rent','home_goods','home_maintenance','transfers','income',
+    'investment','loan_interest','personal_care','pets','gifts','charity','fees','other'));
+
+ALTER TABLE transaction_overrides DROP CONSTRAINT IF EXISTS overrides_category_check;
+ALTER TABLE transaction_overrides ADD CONSTRAINT overrides_category_check
+  CHECK (category_override IS NULL OR category_override IN (
+    'groceries','dining','transport','fuel','shopping','utilities','entertainment',
+    'travel','health','insurance','subscriptions','cash_advance','government',
+    'education','rent','home_goods','home_maintenance','transfers','income',
+    'investment','loan_interest','personal_care','pets','gifts','charity','fees','other'));