Audit constants and make them configurable via process.env

[heyitsaamir]

1. We have some hardcoded constants right now (like in token validation), or proactive messaging. We should make all these easily configurable.
2. It would be great if constants had a single source of truth instead of being pulled in various places from process.env. This makes it hard to track what value it's using. Maybe a config singleton (though I hate singletons), or some dependency injection is worth considering here.