feat(costs): Splitwise-like cost splitting

Add per-payer and per-member custom split amounts with Equally, Custom and
Ticket split modes on top of the existing equal split, keep legacy "paid by"
expenses working, and document the modes in the Budget Tracking wiki page.

server/src/types.ts

@@ -144,6 +144,7 @@ export interface BudgetItemMember {
   avatar_url?: string | null;
   avatar?: string | null;
   budget_item_id?: number;
+  amount?: number | null;
 }
 
 export interface BudgetItemPayer {