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/app/services/cleanup_service.py

@@ -133,6 +133,51 @@ async def apply_deceased(
     return len(persons)
 
 
+# ---- 1b. Mark deceased by a CHILD's birth year -------------------------------------
+# For parents whose own birth date is missing (so the birth-year rule can't reach
+# them) but who have a child born long ago — they're necessarily deceased. Applies
+# through the same apply_deceased() path.
+
+async def preview_deceased_by_child(
+    session: AsyncSession, *, actor: User, tree: Tree, year: int
+) -> list[dict]:
+    await _require_editor(session, actor=actor, tree=tree)
+    names = await _primary_name_by_person(session, tree.id)
+    years = await _birth_year_by_person(session, tree.id)
+    rels = (
+        await session.execute(
+            select(Relationship).where(
+                Relationship.tree_id == tree.id,
+                Relationship.deleted_at.is_(None),
+                Relationship.type == RelationshipType.parent_child,
+            )
+        )
+    ).scalars().all()
+    # parent id -> earliest child birth year, among children born on/before `year`.
+    earliest_child: dict[uuid.UUID, int] = {}
+    for r in rels:
+        cy = years.get(r.person_to_id)  # the child's birth year
+        if cy is None or cy > year:
+            continue
+        if r.person_from_id not in earliest_child or cy < earliest_child[r.person_from_id]:
+            earliest_child[r.person_from_id] = cy
+    persons = {p.id: p for p in await _persons(session, tree.id)}
+    out: list[dict] = []
+    for parent_id, cy in earliest_child.items():
+        p = persons.get(parent_id)
+        if p is None or p.is_living is False:  # gone or already deceased
+            continue
+        out.append(
+            {
+                "person_id": str(parent_id),
+                "name": _display(names.get(parent_id)),
+                "child_birth_year": cy,
+            }
+        )
+    out.sort(key=lambda r: r["child_birth_year"])
+    return out
+
+
 # ---- 2. Re-derive gender from a source GEDCOM (matches by name) ----------------------
 
 async def preview_gender(