Let OpenRouter subtitles use a chosen model instead of whisper-1

A timestamped run on OpenRouter always asked openai/whisper-1 for the
segments, whatever model was picked for plain transcription. Not every
model there returns segment times, so the one to use is now its own
setting, openrouter_subtitle_model, shown in the speech-to-text box only
when OpenRouter is the provider. Empty keeps the old whisper-1 fallback.

Target carries the choice as subtitle_model and timestamp_model() reads
it; the other providers are unchanged.

tests/test_api.py

@@ -53,6 +53,16 @@ class TimestampModel(unittest.TestCase):
         self.assertEqual(api.timestamp_model("openai", "gpt-4o-transcribe"),
                          "whisper-1")
 
+    def test_openrouter_takes_the_subtitle_model_that_was_set(self):
+        self.assertEqual(
+            api.timestamp_model("openrouter", "openai/gpt-4o-transcribe",
+                                "openai/whisper-large-v3"),
+            "openai/whisper-large-v3")
+
+    def test_openrouter_with_no_subtitle_model_falls_back_to_whisper(self):
+        self.assertEqual(api.timestamp_model("openrouter", "openai/gpt-4o-transcribe", ""),
+                         "openai/whisper-1")
+
 
 class Explain(DikteTest):
     def error(self, status):
@@ -318,6 +328,13 @@ class TranscribeSegments(DikteTest):
             api.transcribe_segments(OPENROUTER, self.wav)
         self.assertEqual(multipart_fields(calls[0])["model"], "openai/whisper-1")
 
+    def test_openrouter_asks_for_the_subtitle_model_when_one_is_set(self):
+        target = OPENROUTER._replace(subtitle_model="mistralai/voxtral-mini-transcribe")
+        with fake_urlopen(self.reply([{"start": 0, "end": 1, "text": "hi"}])) as calls:
+            api.transcribe_segments(target, self.wav)
+        self.assertEqual(multipart_fields(calls[0])["model"],
+                         "mistralai/voxtral-mini-transcribe")
+
     def test_groq_stays_on_the_model_it_was_given(self):
         target = GROQ._replace(model="whisper-large-v3")
         with fake_urlopen(self.reply([{"start": 0, "end": 1, "text": "hi"}])) as calls: