Implement StepFun Realtime service and enhance AssistantConfig

- Add new fields to AssistantConfig for realtime interface configuration, including types, values, and secrets.
- Introduce StepFunRealtimeService to handle speech-to-speech processing via WebSocket, integrating STT, LLM, and TTS functionalities.
- Refactor pipeline execution to support a new realtime mode, allowing direct text input processing and immediate responses.
- Update model resource testing to include validation for StepFun Realtime connections.
- Enhance service factory to create realtime services based on configuration settings.
- Modify README documentation to reflect new realtime capabilities and usage instructions.

backend/db/seed_model_resources.sql

@@ -15,7 +15,6 @@ VALUES
   ('model_004', 'SiliconFlow-CosyVoice2-0.5B', 'TTS', 'openai-tts',
    '{"modelId":"FunAudioLLM/CosyVoice2-0.5B","apiUrl":"https://api.siliconflow.cn/v1","voice":"FunAudioLLM/CosyVoice2-0.5B:anna","speed":1.0,"sourceSampleRate":24000}',
    '{"apiKey":"replace-me"}', TRUE, FALSE),
-   '{"apiKey":"replace-me"}', TRUE, FALSE),
   ('model_005', '讯飞语音识别', 'ASR', 'xfyun-asr',
    '{"apiUrl":"https://iat-api.xfyun.cn/v2/iat","language":"zh_cn","domain":"iat","accent":"mandarin","dynamicCorrection":false,"frameSize":1280}',
    '{"appId":"replace-me","apiKey":"replace-me","apiSecret":"replace-me"}', TRUE, TRUE),
@@ -36,13 +35,9 @@ VALUES
    '{"apiKey":"replace-me"}', TRUE, FALSE),
   ('model_011', 'text-embedding-3', 'Embedding', 'openai-embedding',
    '{"modelId":"text-embedding-3-small","apiUrl":"https://api.openai.com/v1/embeddings"}',
+   '{"apiKey":"replace-me"}', TRUE, FALSE),
+  ('model_012', 'StepAudio 2.5 Realtime', 'Realtime', 'stepfun-realtime',
+   '{"modelId":"stepaudio-2.5-realtime","apiUrl":"wss://api.stepfun.com/v1/realtime","voice":"linjiajiejie","inputSampleRate":24000,"outputSampleRate":24000,"prefixPaddingMs":500,"silenceDurationMs":300,"energyAwakenessThreshold":2500}',
    '{"apiKey":"replace-me"}', TRUE, FALSE)
-ON CONFLICT (id) DO UPDATE SET
-  name = EXCLUDED.name,
-  capability = EXCLUDED.capability,
-  interface_type = EXCLUDED.interface_type,
-  values = EXCLUDED.values,
-  secrets = EXCLUDED.secrets,
-  enabled = EXCLUDED.enabled,
-  is_default = EXCLUDED.is_default,
-  updated_at = now();
+-- Seed defaults must never overwrite resources configured through the UI.
+ON CONFLICT (id) DO NOTHING;