refactor services to use Interface and change name to app

internal/service/interfaces.go

@@ -0,0 +1,28 @@
+package service
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/LazyBachelor/LazyPM/internal/models"
+	"github.com/steveyegge/beads"
+)
+
+type App struct {
+	Config Config
+	Issues IssueService
+	Stats  StatsService
+	Logger *slog.Logger
+}
+
+type IssueService interface {
+	beads.Storage
+	AllIssues(ctx context.Context) ([]models.Issue, error)
+	DeleteIssues() error
+}
+
+type StatsService interface {
+	Load(ctx context.Context) error
+	Save(ctx context.Context) error
+	GetStatistics() (models.Statistics, error)
+}