refactor: improve notification naming and introduce type-safe enums

- Rename billFwd* to billsNotification* for clarity
- Rename rentDueNotification* to rentNotification* for consistency
- Rename utilBillsProofOfPayment to billsProofOfPayment
- Introduce enums for type safety:
  - BillsNotificationStrategy (WhenPayed, WhenAttached)
  - BillsNotificationStatus (Scheduled, Sent, Failed)
  - RentNotificationStatus (Sent, Failed)
- Replace "pending" status with "scheduled" for better semantics
- Fix function names to proper camelCase
- Fix incorrect import path in web-app/app/lib/format.ts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/app/lib/format.ts

@@ -1,4 +1,4 @@
-import { YearMonth } from "./db-types";
+import { YearMonth } from "@evidencija-rezija/shared-code";
 
 export const formatYearMonth = ({ year, month }: YearMonth): string => {
   return `${year}-${month<10?"0":""}${month}`;