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/repository/db.go

@@ -127,3 +127,40 @@ func RunMigrations(db *sqlx.DB) error {
 
 	return nil
 }
+
+func RunFinanceMigrations(db *sqlx.DB) error {
+	migrations := []string{
+		`CREATE TABLE IF NOT EXISTS finance_categories (
+			id SERIAL PRIMARY KEY,
+			user_id INTEGER REFERENCES users(id) ON DELETE CASCADE,
+			name VARCHAR(100) NOT NULL,
+			emoji VARCHAR(10) DEFAULT '',
+			type VARCHAR(10) NOT NULL,
+			budget DECIMAL(12,2),
+			color VARCHAR(7) DEFAULT '#6366f1',
+			sort_order INTEGER DEFAULT 0,
+			created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+		)`,
+		`CREATE TABLE IF NOT EXISTS finance_transactions (
+			id SERIAL PRIMARY KEY,
+			user_id INTEGER REFERENCES users(id) ON DELETE CASCADE,
+			category_id INTEGER REFERENCES finance_categories(id) ON DELETE SET NULL,
+			type VARCHAR(10) NOT NULL,
+			amount DECIMAL(12,2) NOT NULL,
+			description TEXT DEFAULT '',
+			date DATE NOT NULL,
+			created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+		)`,
+		`CREATE INDEX IF NOT EXISTS idx_finance_categories_user ON finance_categories(user_id)`,
+		`CREATE INDEX IF NOT EXISTS idx_finance_transactions_user ON finance_transactions(user_id)`,
+		`CREATE INDEX IF NOT EXISTS idx_finance_transactions_date ON finance_transactions(date)`,
+		`CREATE INDEX IF NOT EXISTS idx_finance_transactions_category ON finance_transactions(category_id)`,
+		`CREATE INDEX IF NOT EXISTS idx_finance_transactions_user_date ON finance_transactions(user_id, date)`,
+	}
+	for _, m := range migrations {
+		if _, err := db.Exec(m); err != nil {
+			return err
+		}
+	}
+	return nil
+}