Add GET /api/v1/attachments/{id} endpoint for downloading attachments
through the MCP proxy. This complements the existing upload endpoint and
enables complete attachment workflows via the proxy API.
Add support for uploading file attachments to Grist documents:
- GristClient.upload_attachment() method using multipart/form-data
- upload_attachment tool function with base64 decoding and MIME detection
- Tool registration in server.py
- Comprehensive unit tests (7 new tests)
Returns attachment ID for linking to records via update_records.
Bumps version to 1.3.0.
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.
The Grist API uses PATCH /tables/{table}/columns with a columns array
in the body, not PATCH /tables/{table}/columns/{column_id}. Updated
the endpoint to match the API spec.
