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/server/server.go

@@ -71,7 +71,7 @@ func (s *Server) handleHealth(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(status)
+	_ = json.NewEncoder(w).Encode(status)
 }
 
 func (s *Server) handleReady(w http.ResponseWriter, r *http.Request) {
@@ -89,5 +89,5 @@ func (s *Server) handleReady(w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	json.NewEncoder(w).Encode(status)
+	_ = json.NewEncoder(w).Encode(status)
 }