Replace transactions MVP with ledger entries for cashflow and transfers

[theMattCode]

User story

As a user, I want to record income, expenses, expense reversals, and transfers so my cashflow and account balances stay consistent.

Depends on

• Issue Build finance foundation schema and backend module structure #12
• Issue Add financial account APIs and account management UI #13

Scope

• Replace current transaction API behavior with ledger entry APIs.
• Support income, expense, expense reversal, and transfer entry types.
• Require account and category for future income/expense transactions.
• Support standalone expense reversals with optional related expense link.
• Support transfers between two financial accounts.
• Keep split entry data model usable for future advanced UI.
• Add simple mobile-first Cockpit ledger entry route.
• Preserve the existing /finance/transactions route only if it redirects or is intentionally replaced.

Non-scope

• Full split-entry editing UI.
• Imported candidate matching.
• Budget UI.

Acceptance criteria

• Income and expenses affect cashflow and account balances.
• Expense reversals reduce expense actuals and can stand alone.
• Transfers affect balances but not income, expense, or budget actuals.
• Credit card purchases can be entered as expenses on credit accounts.
• Credit card payments can be entered as transfers.
• Manual ledger entries can be edited and deleted.