Create manual Transaction model

**Epic:** Budget
Labels: epic:budget, type:feature, size:s

**Description**
Add a Transaction model for manual entry and future imports.

**Fields**
- occurred_on (date)
- description (string)
- amount_cents (integer)
- currency (string, default CAD)
- notes (optional)

**Acceptance criteria**
- Belongs to household
- Migration + schema committed