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/models.go

@@ -3366,6 +3366,7 @@ const (
 	ResourceTypeGroupAiBudget               ResourceType = "group_ai_budget"
 	ResourceTypeUserSkill                   ResourceType = "user_skill"
 	ResourceTypeAIGatewayKey                ResourceType = "ai_gateway_key"
+	ResourceTypeUserAiBudgetOverride        ResourceType = "user_ai_budget_override"
 )
 
 func (e *ResourceType) Scan(src interface{}) error {
@@ -3438,7 +3439,8 @@ func (e ResourceType) Valid() bool {
 		ResourceTypeAIProviderKey,
 		ResourceTypeGroupAiBudget,
 		ResourceTypeUserSkill,
-		ResourceTypeAIGatewayKey:
+		ResourceTypeAIGatewayKey,
+		ResourceTypeUserAiBudgetOverride:
 		return true
 	}
 	return false
@@ -3480,6 +3482,7 @@ func AllResourceTypeValues() []ResourceType {
 		ResourceTypeGroupAiBudget,
 		ResourceTypeUserSkill,
 		ResourceTypeAIGatewayKey,
+		ResourceTypeUserAiBudgetOverride,
 	}
 }