feat(api): implement secure portfolio history, daily FX and atomic backups

apps/api/prisma/migrations/202610010002_import_origin/migration.sql

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+-- Additive migration: retain all existing positions and history.
+ALTER TABLE `Position` ADD COLUMN `importedFromId` CHAR(36) NULL;
+CREATE UNIQUE INDEX `Position_userId_importedFromId_key` ON `Position`(`userId`, `importedFromId`);