Create_feedback should accept an author (post on behalf of a user)

Description: Add an optional author argument to createfeedback, accepting a userid or an email, so a post can be attributed to the end user it came from instead of to the authenticated MCP identity. Same reasoning applies to create_comment. Problem: Every post created through MCP is currently authored by whoever holds the token. The originating customer only survives as free-form meta (e.g. {"reporter": "u1024"}), which is not the author field — so the post does not show up under that user, does not appear in their profile or segment, and cannot be filtered with listfeedback's user parameter. Agents ingesting feedback from support channels lose the link between the request and the customer who asked for it. Benefit: Posts created by an agent are indistinguishable in quality from posts created in the portal: right author, right avatar, right user history. Segment- and user-based filtering keeps working for agent-created posts. Follow-up notifications reach the actual requester rather than the token owner. Notes: Behaviour when the email matches no existing user needs deciding: hard error, or create the user implicitly. Should pair with the vote-on-behalf tools (see the related request) so that ingest, attribution and voting share one identity resolution path.