fix: use set_language(None) for auto-detect instead of set_detect_language(true)

detect_language=true causes whisper.cpp to return 0 immediately after
language detection without running the decoder (whisper.cpp source:
  if (params.detect_language) { return 0; }
Setting language=null triggers auto-detection AND transcription.

This was the root cause of 0 segments on all jobs without explicit language.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/transcriber.rs

@@ -90,7 +90,10 @@ impl Transcriber {
         if let Some(lang) = language {
             fp.set_language(Some(lang));
         } else {
-            fp.set_detect_language(true);
+            // Setting language to None (null) triggers whisper's auto-detection AND transcription.
+            // set_detect_language(true) is WRONG: it causes whisper to return 0 after language
+            // detection without transcribing (see whisper.cpp: `if (detect_language) return 0`).
+            fp.set_language(None);
         }
 
         fp.set_translate(task == "translate");