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/lib/provider-brand.ts

@@ -113,6 +113,7 @@ const PROVIDER_BRANDS: Record<string, ProviderBrand> = {
   aihubmix: brand("aihubmix.com", "#111827", "AH"),
   ant_ling: brand("ant-ling.com", "#7C3AED", "AL"),
   anthropic: brand("anthropic.com", "#D97757", "A"),
+  assemblyai: brand("assemblyai.com", "#111827", "AA"),
   atomic_chat: brand("atomic.chat", "#111827", "AC"),
   azure_openai: brand("azure.microsoft.com", "#0078D4", "AZ"),
   bedrock: brand("aws.amazon.com", "#FF9900", "AWS"),