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.

go/internal/io/fio/client.go

@@ -4,6 +4,7 @@ package fio
 
 import (
 	"context"
+	"log/slog"
 	"net/http"
 	"time"
 )
@@ -36,7 +37,9 @@ func New(token, accountNum string, hc httpDoer) Client {
 		hc = http.DefaultClient
 	}
 	if token != "" {
+		slog.Debug("fio: client selected", "type", "api")
 		return &apiClient{token: token, hc: hc}
 	}
+	slog.Debug("fio: client selected", "type", "transparent", "account_num", accountNum)
 	return &transparentClient{accountNum: accountNum, hc: hc}
 }

go/internal/io/fio/transparent.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"log/slog"
 	"net/http"
 	"regexp"
 	"strings"
@@ -28,6 +29,10 @@ func (c *transparentClient) FetchTransactions(ctx context.Context, from, to time
 		from.Format("2.1.2006"),
 		to.Format("2.1.2006"),
 	)
+	slog.Debug("fio transparent: GET",
+		"url", url,
+		"from", from.Format("2006-01-02"), "to", to.Format("2006-01-02"))
+
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
 		return nil, err
@@ -37,6 +42,7 @@ func (c *transparentClient) FetchTransactions(ctx context.Context, from, to time
 		return nil, err
 	}
 	defer resp.Body.Close()
+	slog.Debug("fio transparent: response", "status", resp.StatusCode)
 	if resp.StatusCode != http.StatusOK {
 		return nil, fmt.Errorf("fio transparent: HTTP %d", resp.StatusCode)
 	}
@@ -44,6 +50,7 @@ func (c *transparentClient) FetchTransactions(ctx context.Context, from, to time
 	if err != nil {
 		return nil, err
 	}
+	slog.Debug("fio transparent: body read", "body_bytes", len(body))
 	return parseTransparentHTML(body)
 }
 
@@ -63,6 +70,7 @@ func parseTransparentHTML(body []byte) ([]Transaction, error) {
 	rows := extractSecondTableRows(body)
 
 	var txns []Transaction
+	var droppedBadDate, droppedNonpositive int
 	for _, row := range rows {
 		col := func(i int) string {
 			if i < len(row) {
@@ -72,7 +80,12 @@ func parseTransparentHTML(body []byte) ([]Transaction, error) {
 		}
 		dateStr := parseCzechDate(col(tColDate))
 		amount := parseCzechAmount(col(tColAmount))
-		if dateStr == "" || amount <= 0 {
+		if dateStr == "" {
+			droppedBadDate++
+			continue
+		}
+		if amount <= 0 {
+			droppedNonpositive++
 			continue
 		}
 		txns = append(txns, Transaction{
@@ -86,6 +99,11 @@ func parseTransparentHTML(body []byte) ([]Transaction, error) {
 			BankID:  "", // not available on HTML path
 		})
 	}
+	slog.Debug("fio transparent: parsed",
+		"raw_rows", len(rows),
+		"kept", len(txns),
+		"dropped_bad_date", droppedBadDate,
+		"dropped_nonpositive_amount", droppedNonpositive)
 	return txns, nil
 }