feat: add per-group AI budget table and endpoints (#25203)

Closes
https://linear.app/codercom/issue/AIGOV-284/add-group-budgets-table-and-crud-api

## Summary

Adds the `group_ai_budgets` table and the following endpoints:

- `GET /api/v2/groups/{group}/ai/budget`
- `PUT /api/v2/groups/{group}/ai/budget`
- `DELETE /api/v2/groups/{group}/ai/budget`

Each group may have at most one budget row. If no row exists, no budget
is enforced.

### Feature gate
  
Added `RequireFeatureMW(FeatureAIBridge)` on the `/ai/budget` sub-route.

## RBAC

Authorization reuses `rbac.ResourceGroup` with the existing
`.InOrganization(...).WithID(...)` scoping model.

The `dbauthz` wrappers load the parent `groups` row and authorize
against it.

No new resource type is introduced. As a result, anyone with
`group:update` permissions (Owner, OrgAdmin, or UserAdmin within the
organization) can manage AI budgets for that group.

## Read access for group members

`database.Group.RBACObject()` grants `policy.ActionRead` to all members
of the group through the group ACL:

```go
func (g Group) RBACObject() rbac.Object {
	return rbac.ResourceGroup.WithID(g.ID).
		InOrg(g.OrganizationID).
		// Group members can read the group.
		WithGroupACL(map[string][]policy.Action{
			g.ID.String(): {
				policy.ActionRead,
			},
		})
}
```

Because the `GET` endpoint authorizes against the same loaded `Group`
object, any group member can call:

```text
GET /api/v2/groups/{group}/ai/budget
```

`PUT` and `DELETE` remain admin-only. The group ACL grants only
`ActionRead`, so write operations continue to require role-based
`group:update` permissions.

## Alternative considered

A dedicated `rbac.ResourceGroupAiBudget` resource would allow budget
management to be separated from general group administration.

We decided not to add that complexity for now.

coderd/database/dump.sql

@@ -1765,6 +1765,16 @@ CREATE TABLE gitsshkeys (
     public_key text NOT NULL
 );
 
+CREATE TABLE group_ai_budgets (
+    group_id uuid NOT NULL,
+    spend_limit_micros bigint NOT NULL,
+    created_at timestamp with time zone DEFAULT now() NOT NULL,
+    updated_at timestamp with time zone DEFAULT now() NOT NULL,
+    CONSTRAINT group_ai_budgets_spend_limit_micros_check CHECK ((spend_limit_micros >= 0))
+);
+
+COMMENT ON TABLE group_ai_budgets IS 'Per-group AI spend limit applied to each member of the group. No row means no budget is enforced.';
+
 CREATE TABLE group_members (
     user_id uuid NOT NULL,
     group_id uuid NOT NULL
@@ -3594,6 +3604,9 @@ ALTER TABLE ONLY external_auth_links
 ALTER TABLE ONLY gitsshkeys
     ADD CONSTRAINT gitsshkeys_pkey PRIMARY KEY (user_id);
 
+ALTER TABLE ONLY group_ai_budgets
+    ADD CONSTRAINT group_ai_budgets_pkey PRIMARY KEY (group_id);
+
 ALTER TABLE ONLY group_members
     ADD CONSTRAINT group_members_user_id_group_id_key UNIQUE (user_id, group_id);
 
@@ -4358,6 +4371,9 @@ ALTER TABLE ONLY external_auth_links
 ALTER TABLE ONLY gitsshkeys
     ADD CONSTRAINT gitsshkeys_user_id_fkey FOREIGN KEY (user_id) REFERENCES users(id);
 
+ALTER TABLE ONLY group_ai_budgets
+    ADD CONSTRAINT group_ai_budgets_group_id_fkey FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY group_members
     ADD CONSTRAINT group_members_group_id_fkey FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE;