Implement soft-delete for client targets and balance corrections

Deleting a target or correction sets deletedAt instead of hard-deleting.
Creating a target for a user+client that has a soft-deleted record
reactivates it (clears deletedAt, applies new weeklyHours/startDate)
rather than failing the unique constraint. All reads filter deletedAt = null
on the target, its corrections, and the parent client.

backend/prisma/migrations/20260223210000_add_soft_delete_targets_corrections/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable: add deleted_at column to client_targets
+ALTER TABLE "client_targets" ADD COLUMN "deleted_at" TIMESTAMP(3);
+
+-- AlterTable: add deleted_at column to balance_corrections
+ALTER TABLE "balance_corrections" ADD COLUMN "deleted_at" TIMESTAMP(3);