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

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}"