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/ntfy/client.go

@@ -61,7 +61,7 @@ func (c *Client) Subscribe(ctx context.Context, msgCh chan<- Message) {
 		err := c.connect(ctx, msgCh)
 		if err != nil {
 			if ctx.Err() != nil {
-				return // Context cancelled
+				return // Context canceled
 			}
 			slog.Error("ntfy connection failed", "error", err, "backoff", backoff)
 			time.Sleep(backoff)
@@ -90,7 +90,7 @@ func (c *Client) connect(ctx context.Context, msgCh chan<- Message) error {
 	if err != nil {
 		return fmt.Errorf("connect: %w", err)
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	if resp.StatusCode != http.StatusOK {
 		body, _ := io.ReadAll(io.LimitReader(resp.Body, 1024))