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

updates/202610280006_add_golem15_seo_audit_settings.go

@@ -0,0 +1,41 @@
+package updates
+
+import (
+	"github.com/go-gormigrate/gormigrate/v2"
+	"gorm.io/gorm"
+)
+
+// defaultGenericAnchors is the seed of the generic link texts: the en and
+// pl lists of UI-SPEC C27, one per line.
+const defaultGenericAnchors = "click here\nhere\nmore\nread more\nlearn more\nthis link\nlink\nkliknij tutaj\ntutaj\ntu\nwięcej\nczytaj więcej\nczytaj dalej\nten link"
+
+// addAuditSettings adds the SEO settings' Site audit tab (UI-SPEC C20,
+// D-14): the automatic crawl (off, daily or weekly), pages per crawl, the
+// pause between requests in milliseconds, the crawls kept and the generic
+// link texts, seeded with the en and pl lists.
+var addAuditSettings = []*gormigrate.Migration{
+	{
+		ID: "202610280006_add_golem15_seo_audit_settings",
+		Migrate: func(tx *gorm.DB) error {
+			if err := tx.Exec(`ALTER TABLE golem15_seo_settings
+	ADD COLUMN crawl_schedule TEXT NOT NULL DEFAULT 'off',
+	ADD COLUMN crawl_max_pages INTEGER NOT NULL DEFAULT 5000,
+	ADD COLUMN crawl_delay INTEGER NOT NULL DEFAULT 250,
+	ADD COLUMN crawl_keep INTEGER NOT NULL DEFAULT 10,
+	ADD COLUMN generic_anchors TEXT NOT NULL DEFAULT ''`).Error; err != nil {
+				return err
+			}
+			return tx.Exec(`UPDATE golem15_seo_settings SET generic_anchors = ? WHERE generic_anchors = ''`, defaultGenericAnchors).Error
+		},
+		Rollback: func(tx *gorm.DB) error {
+			return tx.Exec(`ALTER TABLE golem15_seo_settings
+	DROP COLUMN IF EXISTS crawl_schedule, DROP COLUMN IF EXISTS crawl_max_pages,
+	DROP COLUMN IF EXISTS crawl_delay, DROP COLUMN IF EXISTS crawl_keep,
+	DROP COLUMN IF EXISTS generic_anchors`).Error
+		},
+	},
+}
+
+func init() {
+	Register(addAuditSettings...)
+}