fix(api): do the things the API says yes to

Three reports from somebody driving the app through the MCP connector,
all the same shape: an argument accepted, an OK returned, and nothing
done.

**project_id on an update never moved the task.** Both the REST handler
and the MCP tool read it, both checked the caller's rights on the
destination — which reads as "understood" — and then neither passed it
on, because store.TaskUpdate has no such field: a project change is a
move, since sort_order is per project. So the task stayed where it was
and the answer said OK. Moving a task between projects was impossible
except by creating it again and closing the old one, which is what the
person ended up doing. Both paths now go through one helper, so they
cannot drift apart again.

**An unknown #project vanished without a word.** The parser reads
"#3Dekoration" correctly; the lookup then fails, the task goes to the
Inbox, and the name is gone from the title with nothing anywhere saying
why. Still not an error — a thought should not be thrown away over a typo
— but quick-add now returns unknown_project, and the MCP tool returns a
note written for a model to relay. The old behaviour was kind in the web
interface, where you see the row appear in the wrong place and fix it. In
a chat nobody sees the row.

**Sub-tasks were unreachable over MCP.** create_task had no parent_id,
though the REST API has had it all along, so importing a parent with
seven children produced eight unrelated tasks with the parent's name
prefixed onto each. A sub-task joins its parent's project — the two
cannot disagree — and a second level is refused rather than silently
flattened.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/httpapi/integrations_test.go

@@ -983,3 +983,173 @@ func TestWellKnownCalDAVStillRedirects(t *testing.T) {
 		t.Error("CalDAV discovery was caught by the well-known rule")
 	}
 }
+
+// --- what the API accepts, it must actually do -------------------------------------
+
+// project_id on an update used to be validated and then dropped: the destination
+// was permission-checked, which reads as "understood", and the task never moved
+// while the answer said OK. Both the REST path and the MCP tool did it.
+func TestUpdateMovesTheTaskToAnotherProject(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	_, project := ts.do(t, "POST", "/api/v1/projects", map[string]string{"name": "Firma"})
+	projectID := project["id"].(string)
+
+	_, task := ts.do(t, "POST", "/api/v1/tasks", map[string]any{"content": "betal moms"})
+	taskID := task["id"].(string)
+	if task["project_id"] == projectID {
+		t.Fatal("the task started in the destination; the test proves nothing")
+	}
+
+	resp, updated := ts.do(t, "PATCH", "/api/v1/tasks/"+taskID,
+		map[string]any{"project_id": projectID})
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("patch: status %d, body %v", resp.StatusCode, updated)
+	}
+	if updated["project_id"] != projectID {
+		t.Errorf("project_id = %v, want %v — the move was accepted and not made",
+			updated["project_id"], projectID)
+	}
+
+	// And it really is there, not merely reported as there.
+	_, reread := ts.do(t, "GET", "/api/v1/tasks/"+taskID, nil)
+	if reread["project_id"] != projectID {
+		t.Errorf("after re-reading, project_id = %v", reread["project_id"])
+	}
+}
+
+func TestMCPUpdateMovesTheTaskToAnotherProject(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	_, project := ts.do(t, "POST", "/api/v1/projects", map[string]string{"name": "Firma"})
+	projectID := project["id"].(string)
+
+	created := ts.callTool(t, "create_task", map[string]any{"text": "betal moms"})
+	taskID := created["id"].(string)
+
+	moved := ts.callTool(t, "update_task", map[string]any{
+		"task_id": taskID, "project_id": projectID,
+	})
+	if moved["project_id"] != projectID {
+		t.Errorf("project_id = %v, want %v", moved["project_id"], projectID)
+	}
+}
+
+// A project you cannot write to is refused rather than quietly ignored.
+func TestUpdateRefusesAProjectYouCannotWriteTo(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	other := ts.newUser(t, "anden@example.dk", "Anden")
+	_, theirs := other.do(t, "POST", "/api/v1/projects", map[string]string{"name": "Deres"})
+
+	_, task := ts.do(t, "POST", "/api/v1/tasks", map[string]any{"content": "min opgave"})
+	taskID := task["id"].(string)
+	mine := task["project_id"]
+
+	resp, _ := ts.do(t, "PATCH", "/api/v1/tasks/"+taskID,
+		map[string]any{"project_id": theirs["id"]})
+	if resp.StatusCode != http.StatusNotFound {
+		t.Errorf("status %d, want 404", resp.StatusCode)
+	}
+
+	_, reread := ts.do(t, "GET", "/api/v1/tasks/"+taskID, nil)
+	if reread["project_id"] != mine {
+		t.Error("the task moved into somebody else's project")
+	}
+}
+
+// An unknown #project is still not an error — the thought is kept rather than
+// thrown away over a typo — but it is no longer silent. It used to vanish from
+// the title while the task went to the Inbox, with nothing saying why.
+func TestQuickAddSaysWhenTheProjectDoesNotExist(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	resp, body := ts.do(t, "POST", "/api/v1/tasks/quick-add",
+		map[string]any{"text": "Lav plakat #3Dekoration"})
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("status %d, want 201 — the task should still be created", resp.StatusCode)
+	}
+	if body["unknown_project"] != "3Dekoration" {
+		t.Errorf("unknown_project = %v, want %q", body["unknown_project"], "3Dekoration")
+	}
+	if body["content"] != "Lav plakat" {
+		t.Errorf("content = %v", body["content"])
+	}
+}
+
+func TestQuickAddSaysNothingWhenTheProjectExists(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+	ts.do(t, "POST", "/api/v1/projects", map[string]string{"name": "Dekoration"})
+
+	_, body := ts.do(t, "POST", "/api/v1/tasks/quick-add",
+		map[string]any{"text": "Lav plakat #Dekoration"})
+	if _, present := body["unknown_project"]; present {
+		t.Errorf("reported an unknown project for one that exists: %v", body)
+	}
+}
+
+func TestMCPCreateSaysWhenTheProjectDoesNotExist(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	created := ts.callTool(t, "create_task", map[string]any{"text": "Lav plakat #3Dekoration"})
+	note, _ := created["note"].(string)
+	if !strings.Contains(note, "3Dekoration") {
+		t.Errorf("note = %q, want it to name the project that does not exist", note)
+	}
+}
+
+// Sub-tasks were unreachable over MCP: create_task had no parent_id, so a parent
+// with seven children arrived as eight unrelated tasks.
+func TestMCPCreateMakesSubtasks(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	_, project := ts.do(t, "POST", "/api/v1/projects", map[string]string{"name": "Firma"})
+	parent := ts.callTool(t, "create_task", map[string]any{
+		"content": "Runen v2", "project_id": project["id"],
+	})
+	parentID := parent["id"].(string)
+
+	child := ts.callTool(t, "create_task", map[string]any{
+		"content": "Tegn ikonet", "parent_id": parentID,
+	})
+	if child["parent_id"] != parentID {
+		t.Errorf("parent_id = %v, want %v", child["parent_id"], parentID)
+	}
+	// A sub-task joins its parent's project whatever else was asked for.
+	if child["project_id"] != project["id"] {
+		t.Errorf("project_id = %v, want the parent's %v", child["project_id"], project["id"])
+	}
+
+	_, listed := ts.do(t, "GET", "/api/v1/tasks?parent_id="+parentID, nil)
+	if tasks, _ := listed["tasks"].([]any); len(tasks) != 1 {
+		t.Errorf("the parent has %d sub-tasks, want 1", len(tasks))
+	}
+}
+
+func TestMCPCreateRefusesASecondLevelOfSubtask(t *testing.T) {
+	ts := newTestServer(t)
+	ts.bootstrap(t)
+
+	parent := ts.callTool(t, "create_task", map[string]any{"content": "Runen v2"})
+	child := ts.callTool(t, "create_task", map[string]any{
+		"content": "Tegn ikonet", "parent_id": parent["id"],
+	})
+
+	// A tool that refuses reports it inside the result with isError, not as a
+	// JSON-RPC error — the call itself was well formed, the arguments were not.
+	response := ts.rpc(t, "tools/call", map[string]any{
+		"name":      "create_task",
+		"arguments": map[string]any{"content": "Endnu dybere", "parent_id": child["id"]},
+	})
+	result, _ := response["result"].(map[string]any)
+	if result == nil || result["isError"] != true {
+		t.Errorf("a sub-task of a sub-task was accepted: %v", response)
+	}
+}