feat(05-01): migration SQL and Go model types for template system

- Create 002_templates.sql: item_tier enum, ALTER budget_items, templates and template_items tables with CHECK constraint
- Add ItemTier type with fixed/variable/one_off constants to models.go
- Add ItemTier field to BudgetItem struct
- Add Template, TemplateItem, TemplateDetail structs

backend/internal/models/models.go

@@ -18,6 +18,14 @@ const (
 	CategoryIncome          CategoryType = "income"
 )
 
+type ItemTier string
+
+const (
+	ItemTierFixed    ItemTier = "fixed"
+	ItemTierVariable ItemTier = "variable"
+	ItemTierOneOff   ItemTier = "one_off"
+)
+
 type User struct {
 	ID              uuid.UUID `json:"id"`
 	Email           string    `json:"email"`
@@ -58,6 +66,7 @@ type BudgetItem struct {
 	CategoryID     uuid.UUID       `json:"category_id"`
 	CategoryName   string          `json:"category_name,omitempty"`
 	CategoryType   CategoryType    `json:"category_type,omitempty"`
+	ItemTier       ItemTier        `json:"item_tier"`
 	BudgetedAmount decimal.Decimal `json:"budgeted_amount"`
 	ActualAmount   decimal.Decimal `json:"actual_amount"`
 	Notes          string          `json:"notes"`
@@ -86,3 +95,30 @@ type BudgetDetail struct {
 	Items  []BudgetItem `json:"items"`
 	Totals BudgetTotals `json:"totals"`
 }
+
+type Template struct {
+	ID        uuid.UUID `json:"id"`
+	UserID    uuid.UUID `json:"user_id"`
+	Name      string    `json:"name"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+type TemplateItem struct {
+	ID             uuid.UUID        `json:"id"`
+	TemplateID     uuid.UUID        `json:"template_id"`
+	CategoryID     uuid.UUID        `json:"category_id"`
+	CategoryName   string           `json:"category_name,omitempty"`
+	CategoryType   CategoryType     `json:"category_type,omitempty"`
+	CategoryIcon   string           `json:"category_icon,omitempty"`
+	ItemTier       ItemTier         `json:"item_tier"`
+	BudgetedAmount *decimal.Decimal `json:"budgeted_amount"`
+	SortOrder      int              `json:"sort_order"`
+	CreatedAt      time.Time        `json:"created_at"`
+	UpdatedAt      time.Time        `json:"updated_at"`
+}
+
+type TemplateDetail struct {
+	Template
+	Items []TemplateItem `json:"items"`
+}