Cleanup tool: "mark deceased by a child's birth year" rule

Adds a preview/apply rule to the Cleanup tool for parents who have NO birth date
of their own (so the existing born-on-or-before rule can't reach them) but who
have a child born long ago — they're necessarily deceased. This is the gap that
left ~56 parents in the Paul tree as "unknown".

- cleanup_service.preview_deceased_by_child(year): parents of any child born
  on/before the cutoff, excluding already-deceased; returns child_birth_year.
- GET /trees/{id}/cleanup/deceased-by-child?born_on_or_before=1900. Apply reuses
  the existing POST .../cleanup/deceased (same audited mark-deceased path).
- Frontend: a new card in the Cleanup tool (year input → preview → select →
  apply), preview-first like the rest of the tool.

Test covers preview (finds the no-birthdate parent of a pre-cutoff child,
excludes modern-child parents), child_birth_year, apply, and re-preview drop.
Suite 106 passing.

Signed-off-by: Justin Paul <justin@jpaul.me>

backend/tests/test_cleanup.py

@@ -51,6 +51,53 @@ async def test_deceased_preview_and_apply(client):
     assert old not in [r["person_id"] for r in prev2]
 
 
+async def test_deceased_by_child_preview_and_apply(client):
+    h, tid = await _tree(client, "cl-decchild@example.com")
+    # Parent with NO birth date (the gap the birth-year rule can't reach).
+    parent = await _person(client, h, tid, "Gesche", "Frerking")
+    child = await _person(client, h, tid, "Kindt", "Frerking")
+    await _birth(client, h, tid, child, 1880)  # child born before the cutoff
+    await client.post(
+        f"/api/v1/trees/{tid}/relationships",
+        json={"type": "parent_child", "person_from_id": parent, "person_to_id": child},
+        headers=h,
+    )
+    # A parent of a modern child must NOT be flagged.
+    p_modern = await _person(client, h, tid, "Modern", "Parent")
+    c_modern = await _person(client, h, tid, "Kid", "Parent")
+    await _birth(client, h, tid, c_modern, 1990)
+    await client.post(
+        f"/api/v1/trees/{tid}/relationships",
+        json={"type": "parent_child", "person_from_id": p_modern, "person_to_id": c_modern},
+        headers=h,
+    )
+
+    prev = (
+        await client.get(
+            f"/api/v1/trees/{tid}/cleanup/deceased-by-child?born_on_or_before=1900", headers=h
+        )
+    ).json()
+    ids = [r["person_id"] for r in prev]
+    assert parent in ids and p_modern not in ids
+    assert next(r for r in prev if r["person_id"] == parent)["child_birth_year"] == 1880
+
+    # Apply through the shared deceased endpoint.
+    r = await client.post(
+        f"/api/v1/trees/{tid}/cleanup/deceased", json={"person_ids": [parent]}, headers=h
+    )
+    assert r.status_code == 200 and r.json()["updated"] == 1
+    assert (
+        await client.get(f"/api/v1/trees/{tid}/persons/{parent}", headers=h)
+    ).json()["is_living"] is False
+    # Re-preview drops the now-deceased parent.
+    prev2 = (
+        await client.get(
+            f"/api/v1/trees/{tid}/cleanup/deceased-by-child?born_on_or_before=1900", headers=h
+        )
+    ).json()
+    assert parent not in [r["person_id"] for r in prev2]
+
+
 async def test_gender_from_spouse_preview_and_apply(client):
     h, tid = await _tree(client, "cl-spouse@example.com")
     husband = (