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/discord/fuzz_test.go

@@ -116,10 +116,7 @@ func FuzzWebhookPayloadJSON(f *testing.F) {
 		}
 
 		// Marshaling should not panic
-		_, err := json.Marshal(payload)
-		if err != nil {
-			// JSON encoding errors are acceptable for invalid UTF-8
-			// but should not panic
-		}
+		// Marshaling should not panic; JSON encoding errors are acceptable for invalid UTF-8
+		_, _ = json.Marshal(payload)
 	})
 }