Add trending endpoint

Changes:

• Maintains 2 new keys in redis trending_chars and trending_fresh
  • trending_chars - Stores a list of characters in order by how many unique players played them in the past 3 days
  • trending_fresh - A flag which expires every night at 00:00. Expiration indicates the trending list is stale and needs to be regenerated.
• Adds trending endpoint GET /trending/characters
  • Reads trending_chars from redis and returns the characters as a dictionary of character name => trending rank
  • If trending_chars does not exist the server notify the client the data is not yet ready and start a background task to generate the list
  • Starts a background task to check the freshness of the data. If trending_fresh has expired, regenerates the trending list with fresh data

Clients will recieve a list of characters with their trending "rank" starting with 1 as the highest. Clients will not be able to see exact play counts, unique ips, etc.

Tested locally with mongo and redis. Mongo pipeline reused from the discord bot so it should work. Only ever pulling 3 days worth of data at most once per day.