feat: audit user AI budget override mutations (#25745)

Relates to
https://linear.app/codercom/issue/AIGOV-285/add-user-budget-overrides-table-and-crud-api

Adds audit-log support for `user_ai_budget_override` mutations. Without
it, an admin could quietly change a user's per-user spend cap (e.g. from
`$500` to `$50`), reassign it to a different group, or delete it
entirely with no record of who did it.

Both write (`create-or-update`) and delete actions now generate audit
log entries. Unlike group AI budgets, which only track `spend_limit`,
overrides also track `group_name`: an override can be reassigned to a
different attributed group, so that change needs to show up in the diff.
The raw `spend_limit_micros`, IDs, and timestamps are ignored in favor
of the human-readable `spend_limit` and `group_name`.

Depends on #25439.

## Screenshot

<img width="1343" height="514" alt="image"
src="https://github.com/user-attachments/assets/aee30f58-6e81-435e-9bca-5bc98f49d8d3"
/>

coderd/database/modelmethods.go

@@ -102,6 +102,26 @@ func (b GroupAiBudget) Auditable(groupName string) AuditableGroupAiBudget {
 	}
 }
 
+// AuditableUserAiBudgetOverride is the audit-log representation of
+// UserAiBudgetOverride. It enriches the raw record with the username, the
+// attributed group's name, and a human-readable spend limit so audit
+// entries can display meaningful values instead of UUIDs and micros.
+type AuditableUserAiBudgetOverride struct {
+	UserAiBudgetOverride
+	Username   string `json:"username"`
+	GroupName  string `json:"group_name"`
+	SpendLimit string `json:"spend_limit"`
+}
+
+func (o UserAiBudgetOverride) Auditable(username, groupName string) AuditableUserAiBudgetOverride {
+	return AuditableUserAiBudgetOverride{
+		UserAiBudgetOverride: o,
+		Username:             username,
+		GroupName:            groupName,
+		SpendLimit:           fmt.Sprintf("$%.2f", float64(o.SpendLimitMicros)/1_000_000),
+	}
+}
+
 // Auditable returns an object that can be used in audit logs.
 // Covers both group and group member changes.
 func (g Group) Auditable(members []GroupMember) AuditableGroup {