Add default_search to user_settings

Add migration to add a nullable VARCHAR(32) column with a CHECK
constraint restricting values to book, author, series, translator.
Update SQLx query artifacts, handlers, and serializers to read and
write the new field.

.sqlx/query-f208a1162fac1ff6ad8041c4408054d176bf9c967e6dd1a6e659c36a6cdb31f3.json

@@ -0,0 +1,58 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "\n        SELECT id, user_id, last_name, first_name, username, source, default_search\n        FROM user_settings\n        WHERE user_id = $1\n        ",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "id",
+        "type_info": "Int4"
+      },
+      {
+        "ordinal": 1,
+        "name": "user_id",
+        "type_info": "Int8"
+      },
+      {
+        "ordinal": 2,
+        "name": "last_name",
+        "type_info": "Varchar"
+      },
+      {
+        "ordinal": 3,
+        "name": "first_name",
+        "type_info": "Varchar"
+      },
+      {
+        "ordinal": 4,
+        "name": "username",
+        "type_info": "Varchar"
+      },
+      {
+        "ordinal": 5,
+        "name": "source",
+        "type_info": "Varchar"
+      },
+      {
+        "ordinal": 6,
+        "name": "default_search",
+        "type_info": "Varchar"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Int8"
+      ]
+    },
+    "nullable": [
+      false,
+      false,
+      false,
+      false,
+      false,
+      false,
+      true
+    ]
+  },
+  "hash": "f208a1162fac1ff6ad8041c4408054d176bf9c967e6dd1a6e659c36a6cdb31f3"
+}