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_config.py

@@ -220,6 +220,19 @@ class TranscribeTarget(DikteTest):
         self.assertEqual(target.service, "OpenRouter")
         self.assertEqual(target.api_key, "sk-or-test")
         self.assertEqual(target.model, "openai/whisper-1")
+        self.assertEqual(target.subtitle_model, "")
+
+    def test_openrouter_carries_its_subtitle_model(self):
+        conf = self.config(transcribe_provider="openrouter",
+                           openrouter_api_key="sk-or-test",
+                           openrouter_subtitle_model=" openai/whisper-large-v3 ")
+        self.assertEqual(conf.transcribe_target().subtitle_model,
+                         "openai/whisper-large-v3")
+
+    def test_only_openrouter_has_a_subtitle_model(self):
+        conf = self.config(transcribe_provider="openai", openai_api_key="sk-test",
+                           openrouter_subtitle_model="openai/whisper-large-v3")
+        self.assertEqual(conf.transcribe_target().subtitle_model, "")
 
     def test_groq_when_it_is_picked(self):
         conf = self.config(transcribe_provider="groq", groq_api_key="gsk-test",