fix(db): report write errors, multi-step changes all or nothing (FLAW-14)

Store.execInTx runs related statements in one transaction and returns
the first error. Used for deleting and moving folders (with the trash),
shared folders, folder shares and mounts, deleting task lists and
accounts, pinning, and the password reset (new password and the end of
all other links together). Writes whose success the API reports now
pass their error to the client (logout, new password, setup, sharing
mode, new group, restore, unlinking a note block), follow-up tasks and
alarm markers log failures. Remaining ignored writes are listed as
intentional in flaws.md.

Found on the way: DeleteTaskList unlinked all tasks before checking
ownership, so a stranger with the list ID could empty it; viewers could
unlink tasks from note blocks. Both fixed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/concurrency_test.go

@@ -60,3 +60,36 @@ func TestConcurrentWritersDoNotFail(t *testing.T) {
 		t.Fatalf("%d of %d writes failed", failed, 16*15*2)
 	}
 }
+
+func TestMultiStepChangesAreAllOrNothing(t *testing.T) {
+	store, err := NewStore(filepath.Join(t.TempDir(), "atomic.db"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer store.Close()
+	anna, _ := store.CreateUser("anna@example.com", "hash", "Anna", "member")
+	eve, _ := store.CreateUser("eve@example.com", "hash", "Eve", "member")
+	tl, _ := store.CreateTaskList(anna.ID, "", "", "Haushalt", "", "")
+	task, _ := store.CreateThoughtWithDetails(anna.ID, "Fenster", "normal", "manual", "", tl.ID, "todo", "")
+	_, _ = store.UpdateThoughtTriage(anna.ID, task.ID, "keep", "task", "", "", tl.ID)
+
+	// A stranger deleting the list must not empty it (it used to unlink all tasks before checking)
+	if err := store.DeleteTaskList(eve.ID, tl.ID); err == nil {
+		t.Fatal("foreign user must not delete the task list")
+	}
+	if got, _ := store.GetThoughtByID(anna.ID, task.ID); got.TaskListID != tl.ID {
+		t.Fatalf("task was taken out of the list by a stranger")
+	}
+
+	// A failing step rolls back the earlier ones
+	err = store.execInTx(
+		stmt(`UPDATE thoughts SET content = 'geändert' WHERE id = ?`, task.ID),
+		stmt(`UPDATE gibt_es_nicht SET x = 1`),
+	)
+	if err == nil {
+		t.Fatal("expected an error from the second statement")
+	}
+	if got, _ := store.GetThoughtByID(anna.ID, task.ID); got.Content != "Fenster" {
+		t.Fatalf("first step must be rolled back, content is %q", got.Content)
+	}
+}