fix: add golangci-lint config and fix all lint errors

- Add .golangci.yml with v2 config (errcheck, govet, staticcheck, misspell, etc.)
- Fix 32 errcheck issues across config, discord, ntfy, server packages
- Fix misspelling: cancelled → canceled
- Fix staticcheck: use append(slice...) instead of loop
- Fix staticcheck: remove empty error branch
- Use t.Setenv instead of os.Setenv/Unsetenv in tests
- Update CI workflow: add lint job, release tagging, ntfy notifications

internal/ntfy/client_test.go

@@ -80,7 +80,7 @@ func TestClient_Subscribe_ReceivesMessages(t *testing.T) {
 
 		for _, msg := range messages {
 			data, _ := json.Marshal(msg)
-			fmt.Fprintf(w, "%s\n", data)
+			_, _ = fmt.Fprintf(w, "%s\n", data)
 		}
 	}))
 	defer server.Close()
@@ -134,7 +134,7 @@ func TestClient_Subscribe_FilterEvents(t *testing.T) {
 		w.WriteHeader(http.StatusOK)
 		for _, msg := range messages {
 			data, _ := json.Marshal(msg)
-			fmt.Fprintf(w, "%s\n", data)
+			_, _ = fmt.Fprintf(w, "%s\n", data)
 		}
 	}))
 	defer server.Close()
@@ -194,7 +194,7 @@ func TestClient_Subscribe_ContextCancellation(t *testing.T) {
 func TestClient_connect_ServerError(t *testing.T) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusInternalServerError)
-		w.Write([]byte("internal error"))
+		_, _ = w.Write([]byte("internal error"))
 	}))
 	defer server.Close()
 
@@ -218,7 +218,7 @@ func TestClient_connect_URLConstruction(t *testing.T) {
 	defer server.Close()
 
 	client := NewClient(server.URL, []string{"alerts", "updates"})
-	client.connect(context.Background(), make(chan Message))
+	_ = client.connect(context.Background(), make(chan Message))
 
 	expected := "/alerts,updates/json"
 	if requestedURL != expected {