Add audit log infrastructure and GET /v1/audit-logs endpoint

Introduces an append-only audit trail for all user and system actions:
sandbox lifecycle (create/pause/resume/destroy/auto-pause), snapshots,
team rename, API key create/revoke, member add/remove/leave/role_update,
and BYOC host add/delete/marked_down/marked_up.

- New audit_logs table (migration) with team_id, actor, resource,
  action, scope (team|admin), status (success|info|warning|error),
  metadata, and created_at
- AuditLogger (internal/audit) with named fire-and-forget methods per
  event; system actor used for background events (HostMonitor, TTL reaper)
- GET /v1/audit-logs: JWT-only, cursor pagination (max 200), multi-value
  filters for resource_type and action (comma-sep or repeated params);
  members see team-scoped events only, admins/owners see all
- AuthContext extended with APIKeyID + APIKeyName so API key requests
  record meaningful actor identity
- HostMonitor wired with AuditLogger for auto-pause and host marked_down

internal/db/hosts.sql.go

@@ -583,10 +583,13 @@ WHERE id = $1
 `
 
 // Updates last_heartbeat_at and transitions unreachable hosts back to online.
-// Returns 0 if no host was found (deleted).
+// Returns 0 if no host was found (deleted), which the caller treats as 404.
 func (q *Queries) UpdateHostHeartbeatAndStatus(ctx context.Context, id string) (int64, error) {
 	result, err := q.db.Exec(ctx, updateHostHeartbeatAndStatus, id)
-	return result.RowsAffected(), err
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
 }
 
 const updateHostStatus = `-- name: UpdateHostStatus :exec