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/db2sdk/db2sdk.go

@@ -1418,6 +1418,15 @@ func flattenAndSum(sums map[string]int64, prefix string, m map[string]json.RawMe
 	}
 }
 
+func GroupAIBudget(b database.GroupAiBudget) codersdk.GroupAIBudget {
+	return codersdk.GroupAIBudget{
+		GroupID:          b.GroupID,
+		SpendLimitMicros: b.SpendLimitMicros,
+		CreatedAt:        b.CreatedAt,
+		UpdatedAt:        b.UpdatedAt,
+	}
+}
+
 func InvalidatedPresets(invalidatedPresets []database.UpdatePresetsLastInvalidatedAtRow) []codersdk.InvalidatedPreset {
 	var presets []codersdk.InvalidatedPreset
 	for _, p := range invalidatedPresets {