Add monthly budgets and budget allocation actuals

[theMattCode]

User story

As a user, I want monthly category budgets so I can compare planned spending against actual expenses.

Depends on

• Issue Replace transactions MVP with ledger entries for cashflow and transfers #15
• Issue Add global category management with parent rollups #16

Scope

• Add backend APIs for monthly budgets and budget allocations.
• Allocate planned expense amounts to specific expense categories.
• Calculate actuals from expense transactions and expense reversals in the budget month.
• Exclude transfers, balance snapshots, investment buys/sells, and market movement.
• Add Cockpit budget UI for creating/editing monthly allocations.
• Show planned vs actual progress and over-budget categories.
• Roll child categories up to parent reporting groups where configured.

Non-scope

• Automatic carryover.
• Income budgeting.
• Envelope/category balance accounting.

Acceptance criteria

• User can create and edit a budget for one calendar month.
• User can create allocations for specific expense categories.
• Actuals include expenses and expense reversals.
• Transfers do not affect budget actuals.
• Over-budget categories are surfaced clearly.
• Parent category totals are rollups of child allocations/actuals.