feat(go/fio): debug logging via slog at LOG_LEVEL=DEBUG

Wires slog.SetDefault to honour LOG_LEVEL in all CLI commands and adds
debug logs on the Fio fetch path so a silent "fetched 0 transaction(s)"
can be diagnosed without code changes:

- fio.New: which client variant (api/transparent) was selected
- apiClient: GET URL (token redacted as ****), HTTP status, body bytes,
  parsed transaction count
- transparentClient: GET URL, HTTP status, body bytes, plus parser
  stats (raw rows from second table, kept, dropped_bad_date,
  dropped_nonpositive_amount)

Also suppresses the --print-fio-table block when zero transactions were
fetched, so the bare header no longer prints under that condition.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/io/fio/api.go

@@ -5,7 +5,9 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
+	"strings"
 	"time"
 )
 
@@ -25,6 +27,9 @@ func (c *apiClient) FetchTransactions(ctx context.Context, from, to time.Time) (
 	const layout = "2006-01-02"
 	url := fmt.Sprintf("https://fioapi.fio.cz/v1/rest/periods/%s/%s/%s/transactions.json",
 		c.token, from.Format(layout), to.Format(layout))
+	slog.Debug("fio api: GET",
+		"url", strings.Replace(url, c.token, "****", 1),
+		"from", from.Format(layout), "to", to.Format(layout))
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
@@ -35,6 +40,7 @@ func (c *apiClient) FetchTransactions(ctx context.Context, from, to time.Time) (
 		return nil, err
 	}
 	defer resp.Body.Close()
+	slog.Debug("fio api: response", "status", resp.StatusCode)
 	if resp.StatusCode != http.StatusOK {
 		return nil, fmt.Errorf("fio api: HTTP %d", resp.StatusCode)
 	}
@@ -42,7 +48,9 @@ func (c *apiClient) FetchTransactions(ctx context.Context, from, to time.Time) (
 	if err != nil {
 		return nil, err
 	}
-	return parseAPIResponse(body)
+	txns, err := parseAPIResponse(body)
+	slog.Debug("fio api: parsed", "body_bytes", len(body), "parsed_count", len(txns))
+	return txns, err
 }
 
 // fioAPIResponse is the top-level envelope from the Fio JSON API.