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/openapi.json

@@ -2873,6 +2873,64 @@
         }
       }
     },
+    "/api/v1/trees/{tree_id}/cleanup/deceased-by-child": {
+      "get": {
+        "tags": [
+          "cleanup"
+        ],
+        "summary": "Preview Deceased By Child",
+        "description": "People with a child born on/before the cutoff \u2014 necessarily deceased even\nwhen their own birth date is missing. Apply via POST .../cleanup/deceased.",
+        "operationId": "preview_deceased_by_child_api_v1_trees__tree_id__cleanup_deceased_by_child_get",
+        "parameters": [
+          {
+            "name": "tree_id",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string",
+              "format": "uuid",
+              "title": "Tree Id"
+            }
+          },
+          {
+            "name": "born_on_or_before",
+            "in": "query",
+            "required": false,
+            "schema": {
+              "type": "integer",
+              "default": 1900,
+              "title": "Born On Or Before"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Successful Response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/DeceasedByChildCandidate"
+                  },
+                  "title": "Response Preview Deceased By Child Api V1 Trees  Tree Id  Cleanup Deceased By Child Get"
+                }
+              }
+            }
+          },
+          "422": {
+            "description": "Validation Error",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HTTPValidationError"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/trees/{tree_id}/cleanup/gender/preview": {
       "post": {
         "tags": [
@@ -4897,6 +4955,30 @@
         ],
         "title": "DeceasedApply"
       },
+      "DeceasedByChildCandidate": {
+        "properties": {
+          "person_id": {
+            "type": "string",
+            "format": "uuid",
+            "title": "Person Id"
+          },
+          "name": {
+            "type": "string",
+            "title": "Name"
+          },
+          "child_birth_year": {
+            "type": "integer",
+            "title": "Child Birth Year"
+          }
+        },
+        "type": "object",
+        "required": [
+          "person_id",
+          "name",
+          "child_birth_year"
+        ],
+        "title": "DeceasedByChildCandidate"
+      },
       "DeceasedCandidate": {
         "properties": {
           "person_id": {