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/config/fuzz_test.go

@@ -25,10 +25,7 @@ func FuzzParseTopics(f *testing.F) {
 			return
 		}
 
-		topics := make([]string, 0)
-		for _, topic := range splitAndTrim(input) {
-			topics = append(topics, topic)
-		}
+		topics := append([]string{}, splitAndTrim(input)...)
 
 		// Accessing results should not panic
 		_ = len(topics)