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

classes/crawl/job.go

@@ -115,9 +115,25 @@ type Limits struct {
 // DefaultKeep is the number of crawls kept.
 const DefaultKeep = 10
 
-// LoadLimits reads the crawl limits.
+// LoadLimits reads the crawl limits from the Site audit settings; a value
+// outside its range (written around the settings screen) falls back to the
+// default.
 func LoadLimits(db *gorm.DB) (Limits, error) {
-	return Limits{MaxPages: DefaultMaxPages, Delay: DefaultDelay, Keep: DefaultKeep, GenericAnchors: DefaultGenericAnchors}, nil
+	st, err := classes.LoadSettings(db)
+	if err != nil {
+		return Limits{}, err
+	}
+	out := Limits{MaxPages: DefaultMaxPages, Delay: DefaultDelay, Keep: DefaultKeep, GenericAnchors: ParseAnchors(st.GenericAnchors)}
+	if st.CrawlMaxPages >= models.MinCrawlMaxPages && st.CrawlMaxPages <= models.MaxCrawlMaxPages {
+		out.MaxPages = st.CrawlMaxPages
+	}
+	if st.CrawlDelay >= models.MinCrawlDelay && st.CrawlDelay <= models.MaxCrawlDelay {
+		out.Delay = time.Duration(st.CrawlDelay) * time.Millisecond
+	}
+	if st.CrawlKeep >= models.MinCrawlKeep && st.CrawlKeep <= models.MaxCrawlKeep {
+		out.Keep = st.CrawlKeep
+	}
+	return out, nil
 }
 
 // LoadCrawl reads one crawl.
@@ -393,3 +409,16 @@ var ErrNotRunning = errCancelledCrawl
 
 // pluginLog is the crawler's logger (component seo).
 var pluginLog = diary.For("seo")
+
+// ScheduledDue reports whether the nightly entry starts a crawl on day
+// (in the app's location) for the crawl_schedule setting: daily every
+// night, weekly on Mondays, never when off (P33-40).
+func ScheduledDue(schedule string, day time.Time) bool {
+	switch schedule {
+	case models.CrawlDaily:
+		return true
+	case models.CrawlWeekly:
+		return day.Weekday() == time.Monday
+	}
+	return false
+}