fix: reload approved OAuth API keys from DB on startup

[piyushdotcomm]

Problem

After a server restart, settings.API_KEYS is only repopulated from the
API_KEYS environment variable via sync_env_keys_to_db(). OAuth-approved
keys stored in the database are never reloaded into memory.

Since verify_api_key() validates all requests against settings.API_KEYS
(in-memory only), every OAuth user receives a 401 Invalid API key error
after any server restart or redeployment — even if their key is valid in
the database.

Fix

In startup_event() in main.py, after calling sync_env_keys_to_db(db),
query all approved and active API keys from the database and load them into
settings.API_KEYS:

approved_keys = db.query(APIKey).filter(
    APIKey.approved == True,
    APIKey.is_active == True
).all()
for key_obj in approved_keys:
    if key_obj.key not in settings.API_KEYS:
        settings.API_KEYS[key_obj.key] = {
            "name": key_obj.name,
            "can_change_model": key_obj.can_change_model
        }

This ensures all OAuth-issued keys are always available in memory after
startup, regardless of whether the server was restarted.

Fixes #102