Add source identity to transactions so a feed can be re-imported safely

The CSV importer has no idempotency and structurally cannot have any:
batchInsertCSVTransactions assigns row_index = MAX(row_index) + 1, and
row_index is the fifth column of uq_transaction_identity, so the constraint
is guaranteed a fresh value on every run. The same file imported twice
produces two full sets of rows and nothing objects.

Tolerable for the hand-driven bank CSV this was built for. Not tolerable for
any recurring feed, whose windows overlap by design — and not cleanable
afterwards, since every child of transactions is ON DELETE CASCADE, so a
duplicate must be superseded rather than deleted (0023). ~$42k of re-imported
statement rows already show how that goes.

source_ref carries the provider's own key verbatim, enforced by a partial
unique index rather than an application-side ON CONFLICT that a refactor can
quietly drop.

Found while evaluating Frollo (DECISIONS.md ING-11 in the smarthome repo).
That feed is not being ingested — 88% of it duplicates existing statement
coverage — but this gap is real independently of it.

prisma/schema.prisma

@@ -187,6 +187,9 @@ model transactions {
   superseded_by_id        Int?
   principal_amount        Decimal?      @db.Decimal(12, 2)
   interest_amount         Decimal?      @db.Decimal(12, 2)
+  source                  String?       // feed this row came from, e.g. "frollo"; NULL = statement/manual (migration 0028)
+  source_ref              String?       // the provider's own transaction id — idempotency key for re-imports
+  source_account          String?       // account label as the source names it
   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")