feat(backend): bcrypt+jwt auth package and admin seeding

backend/internal/seed/seed.go

@@ -0,0 +1,29 @@
+package seed
+
+import (
+	"context"
+	"log"
+
+	"booklib/internal/auth"
+	"booklib/internal/store"
+)
+
+func Admin(ctx context.Context, s *store.Store, user, pass string) error {
+	if user == "" || pass == "" {
+		return nil
+	}
+	n, err := s.CountUsers(ctx)
+	if err != nil || n > 0 {
+		return err
+	}
+	h, err := auth.HashPassword(pass)
+	if err != nil {
+		return err
+	}
+	if _, err := s.CreateUser(ctx, user, h, "admin"); err != nil {
+		log.Printf("seed admin %q may already exist: %v", user, err)
+		return nil
+	}
+	log.Printf("seeded initial admin user %q", user)
+	return nil
+}