feat: add Prometheus metrics for AI Governance cost control (#27490)

## Description

Adds Prometheus metrics for AI budget cost control, emitted by the
aibridged server under the `cost_control` subsystem (full names are
prefixed `coder_ai_gateway_`).

- `blocked_requests_total` (counter) — labels: `group_id`
- `blocked_users` (gauge) — labels: `group_id`
- `unpriced_requests_total` (counter) — labels: `provider`, `model`
- `enforcement_duration_seconds` (histogram) — labels: `outcome`

## Changes

- Add `GetOverBudgetUsersPerGroup` query (plus dbauthz/dbmetrics/dbmock
wiring) to count over-budget users per effective group.
- Add a background collector that refreshes the `blocked_users` gauge on
an interval, started only when Prometheus is enabled.
- Wire `Metrics` through the aibridged server, coderd API,
`cli/server.go`, and the enterprise AI gateway handler; recording is
nil-safe when metrics are unset.

Closes
https://linear.app/codercom/issue/AIGOV-296/add-prometheus-metrics-for-cost-control

> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by
@ssncferreira

coderd/coderd.go

@@ -47,6 +47,7 @@ import (
 	"github.com/coder/coder/v2/coderd/agentapi/metadatabatcher"
 	"github.com/coder/coder/v2/coderd/aibridge"
 	"github.com/coder/coder/v2/coderd/aibridge/prices"
+	"github.com/coder/coder/v2/coderd/aibridgedserver"
 	"github.com/coder/coder/v2/coderd/aiseats"
 	_ "github.com/coder/coder/v2/coderd/apidoc" // Used for swagger docs.
 	"github.com/coder/coder/v2/coderd/appearance"
@@ -2318,6 +2319,8 @@ type API struct {
 	// routes (license-gated) which apply their own StripPrefix, and by
 	// the in-memory transport (used by chatd, license-exempt).
 	aiGatewayHandler http.Handler
+	// AIGatewayServerMetrics records AI budget cost-control metrics. May be nil.
+	AIGatewayServerMetrics *aibridgedserver.Metrics
 
 	UpdatesProvider tailnet.WorkspaceUpdatesProvider