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/database/dbmetrics/querymetrics.go

@@ -2609,6 +2609,14 @@ func (m queryMetricsStore) GetOrganizationsWithPrebuildStatus(ctx context.Contex
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetOverBudgetUsersPerGroup(ctx context.Context, periodStart time.Time) ([]database.GetOverBudgetUsersPerGroupRow, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetOverBudgetUsersPerGroup(ctx, periodStart)
+	m.queryLatencies.WithLabelValues("GetOverBudgetUsersPerGroup").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetOverBudgetUsersPerGroup").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetParameterSchemasByJobID(ctx context.Context, jobID uuid.UUID) ([]database.ParameterSchema, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetParameterSchemasByJobID(ctx, jobID)