feat: add three-level logging with per-request debug output

- New --log-level flag: debug (default info), info, silent
  debug: every API request logged (method, URL, status, duration)
  info:  normal events (batch progress, entry counts, summaries)
  silent: only warnings and fatal errors
- Replaced all log.Printf/Fatalf calls with level-gated helpers
- API request timing added to queryWikiArticle, queryWikidataBatch, downloadFile
- Retries and backoff logged in debug mode

1 files changed, 78 insertions, 0 deletions

diff --git a/src/loglevel.go b/src/loglevel.go
new file mode 100644
index 0000000..6f0e8aa
--- /dev/null
+++ b/src/loglevel.go
@@ -0,0 +1,78 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+)
+
+// LogLevel controls verbosity.
+type LogLevel int
+
+const (
+	LogDebug LogLevel = iota
+	LogInfo
+	LogSilent
+)
+
+// logLevel is set in main() from the --log-level flag.
+var logLevel LogLevel = LogInfo
+
+// setLogLevel parses the flag value.
+func setLogLevel(val string) {
+	switch val {
+	case "debug":
+		logLevel = LogDebug
+	case "info":
+		logLevel = LogInfo
+	case "silent":
+		logLevel = LogSilent
+	default:
+		log.Fatalf("invalid --log-level %q (want debug, info, or silent)", val)
+	}
+}
+
+func logDebug(format string, v ...interface{}) {
+	if logLevel == LogDebug {
+		log.Printf(format, v...)
+	}
+}
+
+func logInfo(format string, v ...interface{}) {
+	if logLevel <= LogInfo {
+		log.Printf(format, v...)
+	}
+}
+
+func logWarn(format string, v ...interface{}) {
+	log.Printf(format, v...)
+}
+
+func logFatal(format string, v ...interface{}) {
+	log.Fatalf(format, v...)
+}
+
+// logHTTPRequest logs an HTTP request in debug mode.
+func logHTTPRequest(method, url string, statusCode int, duration float64) {
+	if logLevel == LogDebug {
+		log.Printf("HTTP %s %s -> %d (%.2fs)", method, url, statusCode, duration)
+	}
+}
+
+// logHTTPRetry logs a retry attempt in debug mode.
+func logHTTPRetry(attempt, maxAttempts int, backoffSecs int, target string) {
+	if logLevel == LogDebug {
+		log.Printf("retry %d/%d for %s after %ds backoff", attempt, maxAttempts, target, backoffSecs)
+	}
+}
+
+// logFatalErr is like logFatal but takes a message and error separately.
+func logFatalErr(msg string, err error) {
+	log.Fatalf("%s: %v", msg, err)
+}
+
+// Fatal with two args like the original log.Fatal(err, msg) pattern.
+func logFatalTwoArgs(err error, msg string) {
+	fmt.Fprintln(os.Stderr, err)
+	log.Fatalf("%s", msg)
+}