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.

config.yaml.example

@@ -23,6 +23,14 @@ documents:
     doc_id: pN0zE5sT2qP7x
     api_key: ${GRIST_PERSONAL_API_KEY}
 
+  # Docker networking example: connect via internal hostname,
+  # but send the external domain in the Host header
+  docker-grist:
+    url: http://grist:8080
+    doc_id: abc123
+    api_key: ${GRIST_API_KEY}
+    host_header: grist.example.com  # Required when Grist validates Host header
+
 # Agent tokens with access scopes
 tokens:
   - token: REPLACE_WITH_GENERATED_TOKEN

src/grist_mcp/config.py

@@ -14,6 +14,7 @@ class Document:
     url: str
     doc_id: str
     api_key: str
+    host_header: str | None = None  # Override Host header for Docker networking
 
 
 @dataclass
@@ -78,6 +79,7 @@ def load_config(config_path: str) -> Config:
             url=doc_data["url"],
             doc_id=doc_data["doc_id"],
             api_key=doc_data["api_key"],
+            host_header=doc_data.get("host_header"),
         )
 
     # Parse tokens

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: