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/webhook.go

@@ -117,7 +117,7 @@ func (c *Client) Send(ctx context.Context, webhookURL string, msg ntfy.Message)
 	if err != nil {
 		return fmt.Errorf("send request: %w", err)
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	// Handle rate limiting
 	if resp.StatusCode == http.StatusTooManyRequests {