Cleanup: best-guess sex from first name (offline dictionary)

A "Guess from first name" option in the Cleanup gender section: a bundled,
curated given-name -> sex dictionary (weighted English + German for the first
real tree) proposes sex for people who don't have it set. Deterministic, offline,
no model. Genuinely ambiguous names (Marion, Frances, Jordan, …) are excluded
from both sets so they're left for a human. Reuses the existing preview/apply
gender flow, so every guess is reviewed before saving.

No migration. 56 backend tests pass; frontend builds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/openapi.json

@@ -2867,6 +2867,54 @@
         }
       }
     },
+    "/api/v1/trees/{tree_id}/cleanup/gender/guess": {
+      "get": {
+        "tags": [
+          "cleanup"
+        ],
+        "summary": "Guess Gender",
+        "description": "Best-guess sex from first names (bundled dictionary) for people missing it.",
+        "operationId": "guess_gender_api_v1_trees__tree_id__cleanup_gender_guess_get",
+        "parameters": [
+          {
+            "name": "tree_id",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string",
+              "format": "uuid",
+              "title": "Tree Id"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "Successful Response",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/GenderProposal"
+                  },
+                  "title": "Response Guess Gender Api V1 Trees  Tree Id  Cleanup Gender Guess Get"
+                }
+              }
+            }
+          },
+          "422": {
+            "description": "Validation Error",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HTTPValidationError"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/trees/{tree_id}/cleanup/gender": {
       "post": {
         "tags": [