Refactor startup events

[alyssadai]

• Move startup events called in main.py to a dedicated startup.py
• If possible, turn startup checks of environment variables into model validators in env_settings.py:
  

  api/app/main.py

  Lines 42 to 45 in 63de164

  	def validate_environment_variables():
  	"""
  	Check that all required environment variables are set, and exits the app if any are missing or invalid.
  	"""

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[github-actions]

We want to keep our issues up to date and active. This issue hasn't seen any activity in the last 75 days.
We have applied the _flag:stale label to indicate that this issue should be reviewed again.
When you review, please reread the spec and then apply one of these three options:

• prioritize: apply the flag:schedule label to suggest moving this issue into the backlog now
• close: if the issue is no longer relevant, explain why (give others a chance to reply) and then close.
• archive: sometimes an issue has important information or ideas but we won't work on it soon. In this case
  apply the someday label to show that this won't be prioritized. The stalebot will ignore issues with this
  label in the future. Use sparingly!