notification: B3 — Content-Type header + URL trim

Fixes flagged in spec review of 1ff0b2d: ntfy POSTs need explicit
Content-Type: text/plain (the spec calls for it; ntfy works without
but explicit beats inferred); trim trailing slashes from server URL
to avoid double-slash when operators paste 'https://ntfy.sh/'.

internal/notification/ntfy_test.go

@@ -12,12 +12,13 @@ func TestNtfySendsHeadersAndBody(t *testing.T) {
 	t.Parallel()
 
 	var (
-		gotTitle string
-		gotPri   string
-		gotTags  string
-		gotClick string
-		gotAuth  string
-		gotBody  string
+		gotTitle       string
+		gotPri         string
+		gotTags        string
+		gotClick       string
+		gotAuth        string
+		gotContentType string
+		gotBody        string
 	)
 
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -26,6 +27,7 @@ func TestNtfySendsHeadersAndBody(t *testing.T) {
 		gotTags = r.Header.Get("Tags")
 		gotClick = r.Header.Get("Click")
 		gotAuth = r.Header.Get("Authorization")
+		gotContentType = r.Header.Get("Content-Type")
 		b, _ := io.ReadAll(r.Body)
 		gotBody = string(b)
 		w.WriteHeader(http.StatusOK)
@@ -73,6 +75,9 @@ func TestNtfySendsHeadersAndBody(t *testing.T) {
 	if want := "Bearer tk1"; gotAuth != want {
 		t.Errorf("Authorization: got %q want %q", gotAuth, want)
 	}
+	if gotContentType != "text/plain" {
+		t.Errorf("Content-Type: got %q want %q", gotContentType, "text/plain")
+	}
 	if gotBody != "errors found" {
 		t.Errorf("body: got %q want %q", gotBody, "errors found")
 	}