Accept Invite

Accept a workspace invite using the token from the invitation email. The `token` path parameter comes from the invitation link sent to the invitee's email. The caller must be authenticated and their verified email address must match the address the invite was sent to (403 if it does not). On success the caller is added to the workspace with the role specified in the invite, and `workspace_id` is returned so the caller can immediately begin using that workspace. If the caller is already a member of the workspace (e.g. accepted via a different device), the accept is idempotent and returns the same `workspace_id`. Error cases (all return 410 Gone): - Invite already accepted. - Invite was revoked by an admin. - Invite has expired (14-day TTL from creation). The workspace owner's plan seat limit is re-checked at accept time in case the plan was downgraded after the invite was sent; exceeding the limit returns 402.