Create, update and remove votes on behalf of a user

Description: Add write tools for voting to the MCP server: create a vote, update a vote (e.g. upvote → downvote), and remove a vote. Each should identify the voter by user_id or email — ideally both are accepted, with email resolved to an existing user (and a clear error when no user matches). Today the vote surface is read-only: listvotes, getvoters and getfeedbackstats can report votes, but nothing can cast one. There is no way to attribute a vote to a specific end user through MCP at all. Problem: Agents and automations that process incoming feedback (support conversations, Slack, email) can create a post but cannot register the requester's vote on an existing one. The result is duplicate posts instead of accumulated votes on the canonical request, which distorts prioritisation. Merging afterwards is manual cleanup, not a fix. Benefit: "Customer X asked for this too" becomes a single tool call instead of a duplicate post. Vote counts stay an honest signal of demand, because the demand routed through agents lands on the right post. Removing/updating votes allows corrections without touching the database. Open design questions: Permission model: which token scopes may vote as another user, and how impersonation is audited (e.g. a meta.origin marker on agent-cast votes). Abuse surface: an agent-castable vote is trivial to inflate, so rate limiting and/or a distinguishable vote source should be part of the design. Vote directions to support: upvote, downvote, plus whether subscribe and priority follow the same tool shape.