multi-tenant support

Support for multiple hackathons running at once, which wasn't part of the original design intention of vote but is a desired feature.

Start by reading the repository structure and the existing vote data flow; the issue names no files, entry points, or tests. Identify where a single hackathon is assumed and determine the scope needed to support multiple hackathons concurrently. Done should mean the required tenancy behavior and acceptance checks are explicitly defined.

Written by the indexing model from the issue text.