Add recurring plans and confirmation reminders

[theMattCode]

User story

As a user, I want reminders for recurring income, expenses, and transfers so expected money movement can be reviewed before it affects my records.

Depends on

• Issue Replace transactions MVP with ledger entries for cashflow and transfers #15

Scope

• Add backend APIs for recurring plans and reminders.
• Support recurring plan kinds: expected income, expected expense, expected transfer.
• Support monthly, weekly, and yearly recurring schedules.
• Store expected amount, account, category or transfer target, next due date, and reminder lead days.
• Generate upcoming reminders.
• Confirm reminders into actual ledger entries.
• Add Cockpit UI for managing recurring plans and confirming reminders.

Non-scope

• Full custom calendar rule engine.
• Automatic creation of actual ledger entries without confirmation.
• Notification delivery outside the app.

Acceptance criteria

• User can create recurring plans for income, expenses, and transfers.
• Upcoming reminders are generated from recurring schedules.
• Reminders do not affect cashflow or balances before confirmation.
• Confirming a reminder creates the correct ledger entry type.
• Confirmed reminders no longer appear as pending action items.