refactor(discovery/crawler): hoist land constants; document Tribe date format assumption

backend/internal/domain/discovery/crawler/constants.go

@@ -0,0 +1,11 @@
+package crawler
+
+// Land names used in RawEvent/MergedEvent.Land and throughout discovery. ASCII
+// forms (Oesterreich, not Österreich) to keep string comparisons and log output
+// stable across locales. Defined in one place so every source parser references
+// the same constants.
+const (
+	landDeutschland = "Deutschland"
+	landOesterreich = "Oesterreich"
+	landSchweiz     = "Schweiz"
+)

backend/internal/domain/discovery/crawler/marktkalendarium.go

@@ -10,12 +10,6 @@ import (
 	"github.com/PuerkitoBio/goquery"
 )
 
-const (
-	landDeutschland = "Deutschland"
-	landOesterreich = "Oesterreich"
-	landSchweiz     = "Schweiz"
-)
-
 // MarktkalendariumSource scrapes www.marktkalendarium.de. Table rows:
 // Von | Bis | Veranstaltung | Ort | Platz | Webseite | Veranstalter
 type MarktkalendariumSource struct {

backend/internal/domain/discovery/crawler/mittelaltermarkt_online.go

@@ -168,7 +168,14 @@ func tribeCountryToLand(country string) string {
 	return ""
 }
 
-// parseTribeDate parses the Tribe Events date format "YYYY-MM-DD HH:MM:SS" (local time).
+// parseTribeDate parses the Tribe Events JSON date format
+// "YYYY-MM-DD HH:MM:SS" in the site's local time. If the site ever switches
+// to ISO-8601 with T-separator or an explicit timezone, this function will
+// start returning errors and events will be silently dropped. The symptom
+// will show up as "eventsFetched > 0 but Discovered = 0" in CrawlSummary —
+// at that point switch to the iCal endpoint at
+// https://mittelaltermarkt.online/events/?ical=1 which uses the standard
+// VEVENT DTSTART format and is more durable across plugin upgrades.
 func parseTribeDate(s string) (*time.Time, error) {
 	s = strings.TrimSpace(s)
 	if s == "" {