feat: site audit settings and the nightly crawl

- Site audit tab (UI-SPEC C20): crawl_schedule, crawl_max_pages
  (10-50 000), crawl_delay (100-5000 ms), crawl_keep (1-50) and
  generic_anchors seeded with the en and pl lists; ranges refuse with
  lagoon::validation.between on the screen and through the API
- the crawler reads its limits and generic anchors from the settings;
  crawls beyond crawl_keep are deleted with their pages and issues
- seo:crawl [--scheduled] and the nightly schedule entry at 02:30
  (P33-40): off does nothing, weekly runs on Mondays in app.timezone
- TestCrawlSchedule

models/settings/fields.yaml

@@ -411,6 +411,50 @@ fields:
                     type: text
                     span: right
 
+    crawl_schedule:
+        label: golem15.seo::lang.settings.crawl_schedule
+        comment: golem15.seo::lang.settings.crawl_schedule_comment
+        type: dropdown
+        span: left
+        default: "off"
+        options:
+            "off": golem15.seo::lang.settings.crawl_schedule_off
+            daily: golem15.seo::lang.settings.crawl_schedule_daily
+            weekly: golem15.seo::lang.settings.crawl_schedule_weekly
+        tab: golem15.seo::lang.settings.tab_audit
+
+    crawl_max_pages:
+        label: golem15.seo::lang.settings.crawl_max_pages
+        comment: golem15.seo::lang.settings.crawl_max_pages_comment
+        type: number
+        span: right
+        default: 5000
+        tab: golem15.seo::lang.settings.tab_audit
+
+    crawl_delay:
+        label: golem15.seo::lang.settings.crawl_delay
+        comment: golem15.seo::lang.settings.crawl_delay_comment
+        type: number
+        span: left
+        default: 250
+        tab: golem15.seo::lang.settings.tab_audit
+
+    crawl_keep:
+        label: golem15.seo::lang.settings.crawl_keep
+        comment: golem15.seo::lang.settings.crawl_keep_comment
+        type: number
+        span: right
+        default: 10
+        tab: golem15.seo::lang.settings.tab_audit
+
+    generic_anchors:
+        label: golem15.seo::lang.settings.generic_anchors
+        comment: golem15.seo::lang.settings.generic_anchors_comment
+        type: textarea
+        size: small
+        span: full
+        tab: golem15.seo::lang.settings.tab_audit
+
     verify_google:
         label: golem15.seo::lang.settings.verify_google
         comment: golem15.seo::lang.settings.verify_code_comment