feat: add boundary usage tracking database schema and tracker skeleton (#21670)

feat: add boundary usage telemetry database schema and RBAC

Adds the foundation for tracking boundary usage telemetry across Coder
replicas. This includes:

  - Database schema: `boundary_usage_stats` table with per-replica stats
    (unique workspaces, unique users, allowed/denied request counts)
  - Database queries: upsert stats, get aggregated summary, reset stats,
    delete by replica ID
  - RBAC: `boundary_usage` resource type with read/update/delete actions,
    accessible only via system `BoundaryUsageTracker` subject (not regular
    user roles)
  - Tracker skeleton + docs: stub implementation in `coderd/boundaryusage/`

The tracker accumulates stats in memory and periodically flushes to the
database. Stats are aggregated across replicas for telemetry reporting,
then reset when a new reporting period begins. The tracker implementation
and plumbing will be done in a subsequent commit/PR.

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

coderd/rbac/authz.go

@@ -80,6 +80,7 @@ const (
 	SubjectTypeUsagePublisher               SubjectType = "usage_publisher"
 	SubjectAibridged                        SubjectType = "aibridged"
 	SubjectTypeDBPurge                      SubjectType = "dbpurge"
+	SubjectTypeBoundaryUsageTracker         SubjectType = "boundary_usage_tracker"
 )
 
 const (