fix: recurrence ghost exactness and calendar dedup

tests/test_mvp_backend.py

@@ -154,6 +154,120 @@ def test_recurrence_mutations_reject_ghost_occurrences(client):
     assert len([row for row in calendar if row.get("recurrence_id") == recurrence["id"]]) == 3
 
 
+def test_calendar_does_not_duplicate_moved_exceptions_when_both_in_range(client):
+    inbox = boot(client)
+    task = client.post(
+        "/api/v1/tasks",
+        json={"title": "每周一", "list_id": inbox["id"], "due_at": "2026-09-14T09:00:00Z"},
+    ).json()
+    recurrence = client.post(
+        "/api/v1/recurrences", json={"task_id": task["id"], "rrule": "FREQ=WEEKLY;BYDAY=MO;COUNT=4"}
+    ).json()
+    # Series: 9/14, 9/21, 9/28, 10/5. Move the 9/21 occurrence to 9/23.
+    client.patch(
+        f"/api/v1/recurrences/{recurrence['id']}",
+        params={"scope": "this", "occurrence_at": "2026-09-21T09:00:00Z"},
+        json={"due_at": "2026-09-23T09:00:00Z"},
+    )
+    calendar = client.get(
+        "/api/v1/calendar",
+        params={"start": "2026-09-01", "end": "2026-09-30", "timezone": "UTC"},
+    ).json()
+    matches = [row for row in calendar if row.get("recurrence_id") == recurrence["id"]]
+    keys = [(row["occurrence_at"], row["due_at"]) for row in matches]
+    assert len(keys) == len(set(keys))
+    assert len(matches) == 3
+    due_days = [row["due_at"][:10] for row in matches]
+    assert due_days == ["2026-09-14", "2026-09-23", "2026-09-28"]
+    assert not any(row["due_at"].startswith("2026-09-21") for row in matches)
+
+
+def test_recurrence_rejects_wrong_time_of_day_and_alternate_offset(client):
+    inbox = boot(client)
+    task = client.post(
+        "/api/v1/tasks",
+        json={"title": "九点站会", "list_id": inbox["id"], "due_at": "2026-09-07T09:00:00Z"},
+    ).json()
+    recurrence = client.post(
+        "/api/v1/recurrences", json={"task_id": task["id"], "rrule": "FREQ=WEEKLY;BYDAY=MO;COUNT=3"}
+    ).json()
+
+    wrong_hour = client.patch(
+        f"/api/v1/recurrences/{recurrence['id']}",
+        params={"scope": "this", "occurrence_at": "2026-09-07T10:00:00Z"},
+        json={"title": "幽灵"},
+    )
+    assert wrong_hour.status_code == 422
+
+    same_instant = client.patch(
+        f"/api/v1/recurrences/{recurrence['id']}",
+        params={"scope": "this", "occurrence_at": "2026-09-07T17:00:00+08:00"},
+        json={"title": "等价时刻"},
+    )
+    assert same_instant.status_code == 200
+
+
+def test_recurrence_rejects_occurrence_after_cutoff(client):
+    inbox = boot(client)
+    task = client.post(
+        "/api/v1/tasks",
+        json={"title": "每天重复", "list_id": inbox["id"], "due_at": "2026-09-01T09:00:00Z"},
+    ).json()
+    recurrence = client.post(
+        "/api/v1/recurrences", json={"task_id": task["id"], "rrule": "FREQ=DAILY;COUNT=10"}
+    ).json()
+    # Cut the series off at 9/3 (ends_at = 9/3T08:59:59.999999Z)
+    client.patch(
+        f"/api/v1/recurrences/{recurrence['id']}",
+        params={"scope": "this-and-future", "occurrence_at": "2026-09-03T09:00:00Z"},
+        json={},
+    )
+    after_cutoff = client.patch(
+        f"/api/v1/recurrences/{recurrence['id']}",
+        params={"scope": "this", "occurrence_at": "2026-09-05T09:00:00Z"},
+        json={"title": "晚于截止"},
+    )
+    assert after_cutoff.status_code == 422
+
+
+def test_calendar_accepts_legacy_timezone_offset(client):
+    inbox = boot(client)
+    client.post(
+        "/api/v1/tasks",
+        json={"title": "上海边界任务", "list_id": inbox["id"], "due_at": "2026-09-01T16:00:00Z"},
+    )
+    calendar = client.get(
+        "/api/v1/calendar",
+        params={"start": "2026-09-01", "end": "2026-09-30", "timezone_offset": 480},
+    ).json()
+    assert any(row["title"] == "上海边界任务" for row in calendar)
+
+
+def test_calendar_does_not_duplicate_moved_exception_when_original_and_moved_in_range(client):
+    inbox = boot(client)
+    task = client.post(
+        "/api/v1/tasks",
+        json={"title": "每周一", "list_id": inbox["id"], "due_at": "2026-09-14T09:00:00Z"},
+    ).json()
+    recurrence = client.post(
+        "/api/v1/recurrences", json={"task_id": task["id"], "rrule": "FREQ=WEEKLY;BYDAY=MO;COUNT=3"}
+    ).json()
+    # Series: 9/14, 9/21, 9/28 — all inside the requested month.
+    # Move the 9/21 occurrence to 9/23 (also inside). Regression: this used to render twice.
+    client.patch(
+        f"/api/v1/recurrences/{recurrence['id']}",
+        params={"scope": "this", "occurrence_at": "2026-09-21T09:00:00Z"},
+        json={"due_at": "2026-09-23T09:00:00Z"},
+    )
+    calendar = client.get(
+        "/api/v1/calendar",
+        params={"start": "2026-09-01", "end": "2026-09-30", "timezone": "UTC"},
+    ).json()
+    matches = [row for row in calendar if row.get("recurrence_id") == recurrence["id"]]
+    due_days = sorted(row["due_at"][:10] for row in matches)
+    assert due_days == ["2026-09-14", "2026-09-23", "2026-09-28"]
+
+
 def test_habit_logs_support_date_range_filter(client):
     boot(client)
     habit = client.post("/api/v1/habits", json={"name": "跑步", "kind": "boolean", "schedule_type": "daily"}).json()