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/audit/diff.go

@@ -39,6 +39,7 @@ type Auditable interface {
 		database.AIGatewayKey |
 		database.Chat |
 		database.AuditableGroupAiBudget |
+		database.AuditableUserAiBudgetOverride |
 		database.UserSecret |
 		database.UserSkill
 }