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

main.go

@@ -33,7 +33,7 @@ func main() {
 		slog.Error("failed to load config", "error", err)
 		os.Exit(1)
 	}
-	defer cfg.Close()
+	defer func() { _ = cfg.Close() }()
 
 	// Start the bridge
 	b := bridge.New(cfg)