Add `get_attachment` MCP tool for downloading attachment content

Problem


When an MCP client reads a document via the fetch tool, image references in the markdown point to /api/attachments.redirect?id=<uuid>. MCP clients cannot follow these URLs because they require HTTP authentication (session cookie or API key) that is separate from the MCP connection's auth context.

This means AI assistants can read document text but cannot see any images or file attachments.

Proposed solution


Add a get_attachment MCP tool that takes an attachment ID and returns the file content (or a signed download URL) through the already-authenticated MCP session, respecting per-user and per-document access controls.