fix: Critical bugs - LLM client, server imports, error handling

Team 1: Critical Bug Fix Squad

Fixed:
- #1: LLM Client async methods use undefined self.client
  Changed to self._async_client in utils/llm.py
- #3: Server Upload endpoint missing UploadFile/File imports
  Added to server.py imports
- #17: LangGraph workflow error recovery
  - Replaced fake fallback with proper error raising
  - Enabled MemorySaver checkpointing for state persistence
  - Added traceback printing for debugging

opus_orchestrator/utils/llm.py

@@ -113,7 +113,7 @@ class LLMClient:
         if max_tokens:
             payload["max_tokens"] = max_tokens
         
-        response = await self.client.post(
+        response = await self._async_client.post(
             f"{self.base_url}/text/chatcompletion_v2",
             headers=headers,
             json=payload,
@@ -157,7 +157,7 @@ class LLMClient:
         if max_tokens:
             payload["max_tokens"] = max_tokens
         
-        response = await self.client.post(
+        response = await self._async_client.post(
             f"{self.base_url}/chat/completions",
             headers=headers,
             json=payload,