### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

LiteLLM can enforce hard budget limits for keys, teams, and users, but there is no early-warning signal before those limits are hit. This proposes configurable webhook alerts that fire when spend crosses a configurable fraction of `max_budget`.

The idea is to support multiple thresholds per entity type. For example, with thresholds `[0.8, 0.85, 0.95]`, three separate webhook POSTs are sent as an entity's spend reaches 80%, 85%, and 95% of its budget. Each threshold fires at most once per budget period (deduplicated via `DualCache`), and all existing Slack/email alerting keeps working unchanged.

Global defaults would be set per entity type through three new `SlackAlertingArgs` fields (`key_budget_alert_thresholds`, `team_budget_alert_thresholds`, `user_budget_alert_thresholds`), configurable via `general_settings.alerting_args` and exposed in the existing `/alerting/settings` UI page. Keys, teams, and users would each accept a `budget_alert_thresholds` field that overrides the global default for that entity only, editable from the Edit Key, Edit …