feat(transcription): add AssemblyAI as transcription provider

Add AssemblyAI as a third transcription provider option alongside
OpenAI and Groq. AssemblyAI offers better accuracy for certain
audio types (distant voices, noisy environments) and serves as a
reliable fallback when other providers struggle.

Changes:
- Add AssemblyAITranscriptionProvider class in providers/transcription.py
- Add 'assemblyai' option in base channel's transcribe_audio()
- Per-channel configuration via transcriptionProvider in config

Usage:
  Set transcriptionProvider: 'assemblyai' and provide an AssemblyAI
  API key via transcriptionApiKey in the channel config.

webui/src/components/settings/SettingsView.tsx

@@ -779,7 +779,7 @@ export function SettingsView({
   const configuredModelProviderOptions = useMemo(
     () =>
       settings?.providers
-        .filter((provider) => provider.configured)
+        .filter((provider) => provider.configured && provider.model_selectable !== false)
         .map((provider) => ({ name: provider.name, label: provider.label })) ?? [],
     [settings],
   );