Implement soft-delete for clients, projects, and time entries

Replace hard deletes with deletedAt timestamp flags on all three entities.
Deleting a client or project only sets its own deletedAt; child records are
excluded implicitly by filtering on parent deletedAt in every read query.
Raw SQL statistics queries also filter out soft-deleted parents.
FK ON DELETE CASCADE removed from Project→Client and TimeEntry→Project.

backend/src/services/client.service.ts

@@ -5,14 +5,14 @@ import type { CreateClientInput, UpdateClientInput } from "../types";
 export class ClientService {
   async findAll(userId: string) {
     return prisma.client.findMany({
-      where: { userId },
+      where: { userId, deletedAt: null },
       orderBy: { name: "asc" },
     });
   }
 
   async findById(id: string, userId: string) {
     return prisma.client.findFirst({
-      where: { id, userId },
+      where: { id, userId, deletedAt: null },
     });
   }
 
@@ -43,8 +43,9 @@ export class ClientService {
       throw new NotFoundError("Client not found");
     }
 
-    await prisma.client.delete({
+    await prisma.client.update({
       where: { id },
+      data: { deletedAt: new Date() },
     });
   }
 }