feat: add finance module (categories, transactions, summary, analytics)

- model/finance.go: FinanceCategory, FinanceTransaction, Summary, Analytics
- repository/finance.go: CRUD + summary/analytics queries
- service/finance.go: business logic with auto-seed default categories
- handler/finance.go: REST endpoints with owner-only check (user_id=1)
- db.go: finance_categories + finance_transactions migrations
- main.go: register /finance/* routes

Endpoints: GET/POST/PUT/DELETE /finance/categories, /finance/transactions
GET /finance/summary, /finance/analytics

internal/model/finance.go

@@ -0,0 +1,114 @@
+package model
+
+import (
+	"database/sql"
+	"time"
+)
+
+type FinanceCategory struct {
+	ID        int64          `db:"id" json:"id"`
+	UserID    int64          `db:"user_id" json:"user_id"`
+	Name      string         `db:"name" json:"name"`
+	Emoji     string         `db:"emoji" json:"emoji"`
+	Type      string         `db:"type" json:"type"`
+	Budget    sql.NullFloat64 `db:"budget" json:"-"`
+	BudgetVal *float64       `db:"-" json:"budget"`
+	Color     string         `db:"color" json:"color"`
+	SortOrder int            `db:"sort_order" json:"sort_order"`
+	CreatedAt time.Time      `db:"created_at" json:"created_at"`
+}
+
+func (c *FinanceCategory) ProcessForJSON() {
+	if c.Budget.Valid {
+		c.BudgetVal = &c.Budget.Float64
+	}
+}
+
+type FinanceTransaction struct {
+	ID           int64     `db:"id" json:"id"`
+	UserID       int64     `db:"user_id" json:"user_id"`
+	CategoryID   int64     `db:"category_id" json:"category_id"`
+	Type         string    `db:"type" json:"type"`
+	Amount       float64   `db:"amount" json:"amount"`
+	Description  string    `db:"description" json:"description"`
+	Date         time.Time `db:"date" json:"date"`
+	CreatedAt    time.Time `db:"created_at" json:"created_at"`
+	// Joined fields
+	CategoryName  string `db:"category_name" json:"category_name,omitempty"`
+	CategoryEmoji string `db:"category_emoji" json:"category_emoji,omitempty"`
+}
+
+type CreateFinanceCategoryRequest struct {
+	Name      string   `json:"name"`
+	Emoji     string   `json:"emoji,omitempty"`
+	Type      string   `json:"type"`
+	Budget    *float64 `json:"budget,omitempty"`
+	Color     string   `json:"color,omitempty"`
+	SortOrder int      `json:"sort_order,omitempty"`
+}
+
+type UpdateFinanceCategoryRequest struct {
+	Name      *string  `json:"name,omitempty"`
+	Emoji     *string  `json:"emoji,omitempty"`
+	Type      *string  `json:"type,omitempty"`
+	Budget    *float64 `json:"budget,omitempty"`
+	Color     *string  `json:"color,omitempty"`
+	SortOrder *int     `json:"sort_order,omitempty"`
+}
+
+type CreateFinanceTransactionRequest struct {
+	CategoryID  int64   `json:"category_id"`
+	Type        string  `json:"type"`
+	Amount      float64 `json:"amount"`
+	Description string  `json:"description,omitempty"`
+	Date        string  `json:"date"`
+}
+
+type UpdateFinanceTransactionRequest struct {
+	CategoryID  *int64   `json:"category_id,omitempty"`
+	Type        *string  `json:"type,omitempty"`
+	Amount      *float64 `json:"amount,omitempty"`
+	Description *string  `json:"description,omitempty"`
+	Date        *string  `json:"date,omitempty"`
+}
+
+type FinanceSummary struct {
+	Balance      float64              `json:"balance"`
+	TotalIncome  float64              `json:"total_income"`
+	TotalExpense float64              `json:"total_expense"`
+	ByCategory   []CategorySummary    `json:"by_category"`
+	Daily        []DailySummary       `json:"daily"`
+}
+
+type CategorySummary struct {
+	CategoryID   int64   `json:"category_id" db:"category_id"`
+	CategoryName string  `json:"category_name" db:"category_name"`
+	CategoryEmoji string `json:"category_emoji" db:"category_emoji"`
+	Type         string  `json:"type" db:"type"`
+	Amount       float64 `json:"amount" db:"amount"`
+	Percentage   float64 `json:"percentage"`
+	Budget       *float64 `json:"budget,omitempty"`
+}
+
+type DailySummary struct {
+	Date   string  `json:"date" db:"date"`
+	Amount float64 `json:"amount" db:"amount"`
+}
+
+type FinanceAnalytics struct {
+	MonthlyTrend       []MonthlyTrend  `json:"monthly_trend"`
+	AvgDailyExpense    float64         `json:"avg_daily_expense"`
+	ComparisonPrevMonth Comparison     `json:"comparison_prev_month"`
+}
+
+type MonthlyTrend struct {
+	Month   string  `json:"month" db:"month"`
+	Income  float64 `json:"income" db:"income"`
+	Expense float64 `json:"expense" db:"expense"`
+}
+
+type Comparison struct {
+	Current     float64 `json:"current"`
+	Previous    float64 `json:"previous"`
+	DiffPercent float64 `json:"diff_percent"`
+}