DB schema for hackathons #675

Summary

Set up the hackathon table that the map, archive, and streaks all depend on.

Contexts

This table is a hard dependency for some other issues(better to confirm first), so get the schema right before building on top of it.

Tasks

• Create hackathon table per spec (name, dates, lat/lng, city, country, type, platform, url, verified, submitted_by, auto_expires)
• Allow lat/lng to be null for online hackathons
• Add indexes for date range and location queries

Acceptance Criteria

• Migration runs cleanly
• Online and offline hackathons both store correctly
• Tests added
• docs updated

Area

backend