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/service/finance.go

@@ -0,0 +1,167 @@
+package service
+
+import (
+	"database/sql"
+	"time"
+
+	"github.com/daniil/homelab-api/internal/model"
+	"github.com/daniil/homelab-api/internal/repository"
+)
+
+type FinanceService struct {
+	repo *repository.FinanceRepository
+}
+
+func NewFinanceService(repo *repository.FinanceRepository) *FinanceService {
+	return &FinanceService{repo: repo}
+}
+
+// --- Categories ---
+
+func (s *FinanceService) ListCategories(userID int64) ([]model.FinanceCategory, error) {
+	// Auto-seed if user has no categories
+	has, err := s.repo.HasCategories(userID)
+	if err != nil {
+		return nil, err
+	}
+	if !has {
+		if err := s.repo.SeedDefaultCategories(userID); err != nil {
+			return nil, err
+		}
+	}
+
+	cats, err := s.repo.ListCategories(userID)
+	if err != nil {
+		return nil, err
+	}
+	if cats == nil {
+		cats = []model.FinanceCategory{}
+	}
+	return cats, nil
+}
+
+func (s *FinanceService) CreateCategory(userID int64, req *model.CreateFinanceCategoryRequest) (*model.FinanceCategory, error) {
+	cat := &model.FinanceCategory{
+		UserID:    userID,
+		Name:      req.Name,
+		Emoji:     req.Emoji,
+		Type:      req.Type,
+		Color:     defaultString(req.Color, "#6366f1"),
+		SortOrder: req.SortOrder,
+	}
+	if req.Budget != nil {
+		cat.Budget = sql.NullFloat64{Float64: *req.Budget, Valid: true}
+	}
+	if err := s.repo.CreateCategory(cat); err != nil {
+		return nil, err
+	}
+	cat.ProcessForJSON()
+	return cat, nil
+}
+
+func (s *FinanceService) UpdateCategory(id, userID int64, req *model.UpdateFinanceCategoryRequest) (*model.FinanceCategory, error) {
+	cat, err := s.repo.GetCategory(id, userID)
+	if err != nil {
+		return nil, err
+	}
+	if req.Name != nil {
+		cat.Name = *req.Name
+	}
+	if req.Emoji != nil {
+		cat.Emoji = *req.Emoji
+	}
+	if req.Type != nil {
+		cat.Type = *req.Type
+	}
+	if req.Budget != nil {
+		cat.Budget = sql.NullFloat64{Float64: *req.Budget, Valid: true}
+	}
+	if req.Color != nil {
+		cat.Color = *req.Color
+	}
+	if req.SortOrder != nil {
+		cat.SortOrder = *req.SortOrder
+	}
+	if err := s.repo.UpdateCategory(cat); err != nil {
+		return nil, err
+	}
+	cat.ProcessForJSON()
+	return cat, nil
+}
+
+func (s *FinanceService) DeleteCategory(id, userID int64) error {
+	return s.repo.DeleteCategory(id, userID)
+}
+
+// --- Transactions ---
+
+func (s *FinanceService) ListTransactions(userID int64, month, year int, categoryID *int64, txType, search string, limit, offset int) ([]model.FinanceTransaction, error) {
+	txs, err := s.repo.ListTransactions(userID, month, year, categoryID, txType, search, limit, offset)
+	if err != nil {
+		return nil, err
+	}
+	if txs == nil {
+		txs = []model.FinanceTransaction{}
+	}
+	return txs, nil
+}
+
+func (s *FinanceService) CreateTransaction(userID int64, req *model.CreateFinanceTransactionRequest) (*model.FinanceTransaction, error) {
+	date, err := time.Parse("2006-01-02", req.Date)
+	if err != nil {
+		date = time.Now()
+	}
+	tx := &model.FinanceTransaction{
+		UserID:      userID,
+		CategoryID:  req.CategoryID,
+		Type:        req.Type,
+		Amount:      req.Amount,
+		Description: req.Description,
+		Date:        date,
+	}
+	if err := s.repo.CreateTransaction(tx); err != nil {
+		return nil, err
+	}
+	return tx, nil
+}
+
+func (s *FinanceService) UpdateTransaction(id, userID int64, req *model.UpdateFinanceTransactionRequest) (*model.FinanceTransaction, error) {
+	tx, err := s.repo.GetTransaction(id, userID)
+	if err != nil {
+		return nil, err
+	}
+	if req.CategoryID != nil {
+		tx.CategoryID = *req.CategoryID
+	}
+	if req.Type != nil {
+		tx.Type = *req.Type
+	}
+	if req.Amount != nil {
+		tx.Amount = *req.Amount
+	}
+	if req.Description != nil {
+		tx.Description = *req.Description
+	}
+	if req.Date != nil {
+		parsed, err := time.Parse("2006-01-02", *req.Date)
+		if err == nil {
+			tx.Date = parsed
+		}
+	}
+	if err := s.repo.UpdateTransaction(tx); err != nil {
+		return nil, err
+	}
+	return tx, nil
+}
+
+func (s *FinanceService) DeleteTransaction(id, userID int64) error {
+	return s.repo.DeleteTransaction(id, userID)
+}
+
+func (s *FinanceService) GetSummary(userID int64, month, year int) (*model.FinanceSummary, error) {
+	return s.repo.GetSummary(userID, month, year)
+}
+
+func (s *FinanceService) GetAnalytics(userID int64, months int) (*model.FinanceAnalytics, error) {
+	return s.repo.GetAnalytics(userID, months)
+}