feat: add OpencodeGo AI provider (deepseek-v4-flash) as primary, existing providers as fallback

backend/app/ai/providers/__init__.py

@@ -4,5 +4,6 @@ from .deepl import DeepLProvider
 from .local import LocalProvider
 from .spark import SparkProvider
 from .sensenova import SensenovaProvider
+from .opencode_go import OpencodeGoProvider
 
-__all__ = ["OpenAIProvider", "ClaudeProvider", "DeepLProvider", "LocalProvider", "SparkProvider", "SensenovaProvider"]
+__all__ = ["OpenAIProvider", "ClaudeProvider", "DeepLProvider", "LocalProvider", "SparkProvider", "SensenovaProvider", "OpencodeGoProvider"]

backend/app/ai/providers/opencode_go.py

@@ -0,0 +1,7 @@
+from app.ai.providers.openai import OpenAIProvider
+
+
+class OpencodeGoProvider(OpenAIProvider):
+    def __init__(self, api_key: str, model: str = "deepseek-v4-flash", base_url: str = "https://opencode.ai/zen/go/v1"):
+        super().__init__(api_key=api_key, model=model, base_url=base_url)
+        self._name = f"opencode-go-{model}"