fix: raise EnvironmentError when WALLET_PRIV_KEY is not set

[bigeez]

Bug

WALLET_PRIV_KEY in server/config.py is typed as str but
os.getenv() returns None if the variable is not set. This None
value is then passed to og.LLM(private_key=...) at module import
time, causing a cryptic crash with no clear explanation.

Fix

Added an explicit check after loading the env var that raises a clear
EnvironmentError with a helpful message if WALLET_PRIV_KEY is missing,
so developers know exactly what to fix instead of hunting down a confusing
traceback.

Impact

• Prevents silent None being passed to the LLM client
• Fails fast with a clear error message at startup
• Makes missing config easy to diagnose