Resolves cache configuration at startup and stores it in :persistent_term for O(1) access.

Called once by EDA.Cache.Supervisor before cache processes start.

Example

config :eda,
  cache: [
    guilds: [],
    users: [max_size: 100_000],
    members: [policy: :none],
    presences: [policy: :none],
    channels: [policy: fn _entity, _key, ch ->
      if ch["type"] in [0, 2, 5], do: :cache, else: :skip
    end]
  ]