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>

frontend/app/trees/[id]/cleanup/page.tsx

@@ -10,6 +10,7 @@ import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
 import { Input } from "@/components/ui/input";
 
 type Deceased = components["schemas"]["DeceasedCandidate"];
+type DeceasedByChild = components["schemas"]["DeceasedByChildCandidate"];
 type GenderProp = components["schemas"]["GenderProposal"];
 type NameIssue = components["schemas"]["NameIssue"];
 type Person = components["schemas"]["PersonRead"];
@@ -31,6 +32,12 @@ export default function CleanupPage() {
   const [decSel, setDecSel] = useState<Set<string>>(new Set());
   const [decMsg, setDecMsg] = useState<string | null>(null);
 
+  // 1b) Deceased by a child's birth year (for parents with no birth date)
+  const [childYear, setChildYear] = useState(1900);
+  const [decByChild, setDecByChild] = useState<DeceasedByChild[] | null>(null);
+  const [dbcSel, setDbcSel] = useState<Set<string>>(new Set());
+  const [dbcMsg, setDbcMsg] = useState<string | null>(null);
+
   // 2) Gender from source GEDCOM
   const [gender, setGender] = useState<GenderProp[] | null>(null);
   const [genSel, setGenSel] = useState<Set<string>>(new Set());
@@ -63,6 +70,23 @@ export default function CleanupPage() {
     setDeceased(null);
   }
 
+  async function previewDeceasedByChild() {
+    setDbcMsg(null);
+    const { data } = await api.GET("/api/v1/trees/{tree_id}/cleanup/deceased-by-child", {
+      params: { path: { tree_id: treeId }, query: { born_on_or_before: childYear } },
+    });
+    setDecByChild(data ?? []);
+    setDbcSel(new Set((data ?? []).map((d) => d.person_id)));
+  }
+  async function applyDeceasedByChild() {
+    const { data } = await api.POST("/api/v1/trees/{tree_id}/cleanup/deceased", {
+      params: { path: { tree_id: treeId } },
+      body: { person_ids: [...dbcSel] },
+    });
+    setDbcMsg(`Marked ${data?.updated ?? 0} people deceased.`);
+    setDecByChild(null);
+  }
+
   async function previewGender(e: React.ChangeEvent<HTMLInputElement>) {
     const file = e.target.files?.[0];
     if (genFile.current) genFile.current.value = "";
@@ -231,6 +255,59 @@ export default function CleanupPage() {
         </CardContent>
       </Card>
 
+      {/* 1b) Deceased by a child's birth year */}
+      <Card>
+        <CardHeader>
+          <CardTitle className="text-base">Mark deceased by a child’s birth year</CardTitle>
+        </CardHeader>
+        <CardContent className="space-y-3">
+          <p className="text-sm text-[var(--muted)]">
+            Catches parents who have <strong>no birth date of their own</strong> (so the rule
+            above can’t reach them) but who have a child born long ago — they’re necessarily
+            deceased.
+          </p>
+          <div className="flex flex-wrap items-end gap-2">
+            <label className="flex flex-col gap-1 text-sm">
+              <span className="text-xs text-[var(--muted)]">Has a child born on or before</span>
+              <Input
+                type="number"
+                className="w-28"
+                value={childYear}
+                onChange={(e) => setChildYear(Number(e.target.value))}
+              />
+            </label>
+            <Button variant="outline" onClick={previewDeceasedByChild}>
+              Preview
+            </Button>
+          </div>
+          {dbcMsg && <p className="text-sm text-bronze">{dbcMsg}</p>}
+          {decByChild && (
+            <div className="space-y-2">
+              <p className="text-sm text-[var(--muted)]">
+                {decByChild.length} people with a child born ≤ {childYear} (not already marked
+                deceased).
+              </p>
+              <ul className="max-h-64 divide-y divide-[var(--border)] overflow-auto rounded-lg border border-[var(--border)]">
+                {decByChild.map((d) => (
+                  <li key={d.person_id} className="flex items-center gap-3 px-3 py-1.5 text-sm">
+                    <input
+                      type="checkbox"
+                      checked={dbcSel.has(d.person_id)}
+                      onChange={() => toggle(dbcSel, d.person_id, setDbcSel)}
+                    />
+                    <span className="flex-1">{d.name}</span>
+                    <span className="text-xs text-[var(--muted)]">child b. {d.child_birth_year}</span>
+                  </li>
+                ))}
+              </ul>
+              {decByChild.length > 0 && (
+                <Button onClick={applyDeceasedByChild}>Mark {dbcSel.size} deceased</Button>
+              )}
+            </div>
+          )}
+        </CardContent>
+      </Card>
+
       {/* 2) Gender from source */}
       <Card>
         <CardHeader>