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/cmd/fuj/main.go

@@ -11,6 +11,7 @@ import (
 	"fuj-management/go/internal/services/banksync"
 	"fuj-management/go/internal/services/membership"
 	"fuj-management/go/internal/web"
+	"log/slog"
 	"os"
 	"time"
 )
@@ -28,6 +29,9 @@ func main() {
 		os.Exit(2)
 	}
 
+	// Honour LOG_LEVEL for slog calls in any package (e.g. internal/io/fio debug logs).
+	slog.SetDefault(logging.New(os.Getenv("LOG_LEVEL")))
+
 	cmd, args := os.Args[1], os.Args[2:]
 
 	switch cmd {