fix: fetch only alerts for unacknowledged events

[blenzi]

Following https://github.com/pyronear/pyro-api/pull/268, this PR fixes the retrieval of alerts by fetching the alerts associated with unacknowledged events. In addition:

• increase the number of maximum events from 5 to 10
• add call_api method and use to avoid code repetition
• protection against undefined variables (raise ValueError)