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/apidoc/docs.go

@@ -22822,6 +22822,7 @@ const docTemplate = `{
                 "ai_provider_key",
                 "ai_gateway_key",
                 "group_ai_budget",
+                "user_ai_budget_override",
                 "chat",
                 "user_secret",
                 "user_skill"
@@ -22858,6 +22859,7 @@ const docTemplate = `{
                 "ResourceTypeAIProviderKey",
                 "ResourceTypeAIGatewayKey",
                 "ResourceTypeGroupAIBudget",
+                "ResourceTypeUserAIBudgetOverride",
                 "ResourceTypeChat",
                 "ResourceTypeUserSecret",
                 "ResourceTypeUserSkill"

coderd/apidoc/swagger.json

@@ -20929,6 +20929,7 @@
 				"ai_provider_key",
 				"ai_gateway_key",
 				"group_ai_budget",
+				"user_ai_budget_override",
 				"chat",
 				"user_secret",
 				"user_skill"
@@ -20965,6 +20966,7 @@
 				"ResourceTypeAIProviderKey",
 				"ResourceTypeAIGatewayKey",
 				"ResourceTypeGroupAIBudget",
+				"ResourceTypeUserAIBudgetOverride",
 				"ResourceTypeChat",
 				"ResourceTypeUserSecret",
 				"ResourceTypeUserSkill"