feat(discovery/crawler): PLZ-to-land inference helper

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

@@ -0,0 +1,45 @@
+package crawler
+
+import "strings"
+
+// InferLand maps a PLZ to a DACH country. 5-digit -> DE. 4-digit is ambiguous
+// between AT and CH; we recognize common CH ranges (1000-1299 Geneva/Vaud,
+// 3000-3999 Bern, 4000-4999 Basel/Aarau, 6000-6999 Luzern/Ticino/Wallis,
+// 8000-8999 Zurich, 9000-9999 St. Gallen) and fall back to AT for everything
+// else 4-digit. Unknown -> "". The Land string is the form used throughout
+// discovery (not ISO-2).
+func InferLand(plz string) string {
+	plz = strings.TrimSpace(plz)
+	switch len(plz) {
+	case 5:
+		if !isAllDigits(plz) {
+			return ""
+		}
+		return "Deutschland"
+	case 4:
+		if !isAllDigits(plz) {
+			return ""
+		}
+		switch plz[0] {
+		case '1':
+			if plz >= "1200" && plz <= "1299" {
+				return "Schweiz"
+			}
+			return "Oesterreich"
+		case '3', '4', '6', '8', '9':
+			return "Schweiz"
+		default:
+			return "Oesterreich"
+		}
+	}
+	return ""
+}
+
+func isAllDigits(s string) bool {
+	for _, r := range s {
+		if r < '0' || r > '9' {
+			return false
+		}
+	}
+	return true
+}

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

@@ -0,0 +1,29 @@
+package crawler
+
+import "testing"
+
+func TestInferLand(t *testing.T) {
+	tests := []struct {
+		name string
+		plz  string
+		want string
+	}{
+		{"empty", "", ""},
+		{"de 5-digit", "49186", "Deutschland"},
+		{"de 5-digit low", "01067", "Deutschland"},
+		{"at 4-digit typical", "1010", "Oesterreich"},
+		{"ch 4-digit typical", "8001", "Schweiz"},
+		{"ch 4-digit zurich range", "8000", "Schweiz"},
+		{"ch 4-digit bern range", "3000", "Schweiz"},
+		{"at 4-digit outside ch ranges", "2500", "Oesterreich"},
+		{"short garbage", "12", ""},
+		{"non-numeric", "abcde", ""},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := InferLand(tc.plz); got != tc.want {
+				t.Errorf("InferLand(%q) = %q; want %q", tc.plz, got, tc.want)
+			}
+		})
+	}
+}