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/tests/provider-brand.test.ts

@@ -47,4 +47,9 @@ describe("provider brand logos", () => {
     expect(providerBrand("openrouter")?.logoUrls).toContain("https://openrouter.ai/favicon.ico");
     expect(providerBrand("openrouter")?.initials).toBe("OR");
   });
+
+  it("keeps AssemblyAI voice settings on the first-party brand domain", () => {
+    expect(providerBrand("assemblyai")?.logoUrls).toContain("https://assemblyai.com/favicon.ico");
+    expect(providerBrand("assemblyai")?.initials).toBe("AA");
+  });
 });