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.

frontend/src/types/index.ts

@@ -152,6 +152,7 @@ export interface BalanceCorrection {
   hours: number;
   description: string | null;
   createdAt: string;
+  deletedAt: string | null;
 }
 
 export interface WeekBalance {