fix(transactions): supersede rows imported twice instead of deleting them

Statements 107, 142 and 143 bill overlapping periods on one ANZ account, so 31
transactions -- $42,040.68 -- are in the ledger twice.

They are marked superseded, not deleted. Every child of transactions is ON
DELETE CASCADE (splits, tags, overrides, expense_metadata, order_reviews), so
deleting "the duplicate" destroys whatever curation sits on it, and which
member of a pair holds that curation is an accident of import order: here 1
pair carries splits and 6 carry overrides, all on the surviving side, but
nothing guarantees that. Superseding keeps the row, keeps its children, and
makes a mistake one UPDATE to undo rather than a restore from backup.

reconciled_with_id could not be reused. Its predicate is scoped to
statement_id IS NULL on purpose -- a statement line pointing at something else
is the survivor, not the duplicate -- and here both rows are statement lines.

The exclusion goes into EXCLUDE_RECONCILED_SOURCE rather than into a new
fragment, so every query already asking "count each purchase once" gets it
without being edited. The trip cost queries did not use that fragment at all
and now do; verified a no-op on current data (0 trip-tagged rows are either
reconciled sources or duplicates), but they were one import away from
double-counting.

Most of the $42k is transfers and investments, which spend already excludes.
The damage was elsewhere: duplicated rows in the list, and rules re-splitting a
duplicate -- txn 3807 is one of these 31 and was a candidate for splitting
earlier today.

Balances are unchanged: no duplicate carried a split.

prisma/migrations/0023_supersede_duplicates/migration.sql

@@ -0,0 +1,37 @@
+-- A transaction imported twice cannot simply be deleted.
+--
+-- Every child of `transactions` is ON DELETE CASCADE — splits, tags, overrides,
+-- expense_metadata, order_reviews. Deleting a row said to be "the duplicate"
+-- therefore destroys whatever curation happens to sit on it, silently and
+-- unrecoverably. The curation is not reliably on the surviving side either: of
+-- the 31 known duplicate pairs, one carries splits and six carry overrides, and
+-- which member holds them is an accident of import order.
+--
+-- So a duplicate is superseded, never removed. The row stays, keeps its
+-- children, and points at the row that replaces it. Reversing a mistake is then
+-- one UPDATE rather than a restore from backup.
+--
+-- This is the statement-vs-statement case. `reconciled_with_id` already covers
+-- manual-vs-statement, and deliberately cannot be reused: the predicate that
+-- hides a reconciled row is scoped to `statement_id IS NULL`, because a
+-- statement line pointing at something else is the survivor, not the duplicate.
+-- Both of these rows are statement lines.
+
+ALTER TABLE transactions
+  ADD COLUMN IF NOT EXISTS superseded_by_id integer
+    REFERENCES transactions(id) ON DELETE SET NULL;
+
+COMMENT ON COLUMN transactions.superseded_by_id IS
+  'This row was imported twice; the named row is the one that counts. Excluded from every figure, kept for its children and its audit trail. NULL = live.';
+
+CREATE INDEX IF NOT EXISTS idx_transactions_superseded
+    ON transactions (superseded_by_id)
+ WHERE superseded_by_id IS NOT NULL;
+
+-- A row cannot supersede itself, and a survivor cannot itself be superseded
+-- (that would hide both members of the pair and lose the amount entirely).
+ALTER TABLE transactions
+  DROP CONSTRAINT IF EXISTS transactions_no_self_supersede;
+ALTER TABLE transactions
+  ADD CONSTRAINT transactions_no_self_supersede
+  CHECK (superseded_by_id IS NULL OR superseded_by_id <> id);

prisma/schema.prisma

@@ -183,11 +183,14 @@ model transactions {
   payment_method          String?       // card | cash | bank_transfer | other; NULL = unknown (migration 0016)
   owner_id                Int?
   reconciled_with_id      Int?
+  superseded_by_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")
+  superseded_by           transactions?  @relation("superseded", fields: [superseded_by_id], references: [id], onDelete: SetNull)
+  supersedes              transactions[] @relation("superseded")
   expense_metadata        expense_metadata?
   order_review            order_reviews?
 }