feat: add host_header config for Docker networking

When Grist validates the Host header (common with reverse proxy setups),
internal Docker networking fails because requests arrive with
Host: container-name instead of the external domain.

The new host_header config option allows overriding the Host header
sent to Grist while still connecting via internal Docker hostnames.

src/grist_mcp/grist_client.py

@@ -17,6 +17,8 @@ class GristClient:
         self._doc = document
         self._base_url = f"{document.url.rstrip('/')}/api/docs/{document.doc_id}"
         self._headers = {"Authorization": f"Bearer {document.api_key}"}
+        if document.host_header:
+            self._headers["Host"] = document.host_header
         self._timeout = timeout
 
     async def _request(self, method: str, path: str, **kwargs) -> dict: