chore: smoke tmpdir hygiene, seed pw floor, README

backend/internal/seed/seed.go

@@ -15,6 +15,10 @@ func Admin(ctx context.Context, s *store.Store, user, pass string) error {
 	if user == "" || pass == "" {
 		return nil
 	}
+	if len(pass) < 8 { // 与 API 建户口令下限一致
+		log.Printf("seed admin skipped: ADMIN_PASSWORD must be >= 8 chars")
+		return nil
+	}
 	n, err := s.CountUsers(ctx)
 	if err != nil || n > 0 {
 		return err

backend/internal/seed/seed_test.go

@@ -21,10 +21,16 @@ func TestSeedOnlyWhenEmpty(t *testing.T) {
 	}
 	s := store.New(p)
 	p.Exec(ctx, "DELETE FROM reading_progress; DELETE FROM books; DELETE FROM libraries; DELETE FROM users")
-	if err := Admin(ctx, s, "admin", "pw12345"); err != nil {
+	if err := Admin(ctx, s, "shorty", "pw123"); err != nil { // <8 位 → 跳过 + log,不报错(空库时验证确实没建)
+		t.Fatal("short pw must no-op, got", err)
+	}
+	if _, err := s.GetUserByName(ctx, "shorty"); err == nil {
+		t.Fatal("short pw must not create user")
+	}
+	if err := Admin(ctx, s, "admin", "pw123456"); err != nil {
 		t.Fatal(err)
 	}
-	if err := Admin(ctx, s, "admin2", "pw12345"); err != nil { // 已有用户 → no-op
+	if err := Admin(ctx, s, "admin2", "pw123456"); err != nil { // 已有用户 → no-op
 		t.Fatal(err)
 	}
 	n, _ := s.CountUsers(ctx)