P1 — Active expiry never runs for non-default namespaces

Heartbeat expiry and eviction scan only the default namespace
(`engine_shard.cc:784-785`, `862-864`), and the post-expiry wake dispatch uses only the
default namespace's `BlockingController` (`engine_shard.cc:959-964`). Keys in ACL
namespaces are never actively expired at all — no deletion, no keyspace notification, no
wake. Only a later tenant command that touches the key expires it lazily.

Scenario:

```text
ACL SETUSER u NAMESPACE:tenant ON >p +@all ~*

# tenant connection 1
XGROUP CREATE s g 0 MKSTREAM
XREADGROUP GROUP g c BLOCK 0 STREAMS s >

# tenant connection 2
PEXPIRE s 100
```

Actual: the blocked tenant client is never woken; with `BLOCK 0` it stays blocked
indefinitely. A later tenant command touching `s` (even `EXISTS s`) lazily expires the key
and only then the reader receives `NOGROUP`. The identical scenario in the default
namespace correctly delivers `NOGROUP` shortly after the TTL elapses.

Start in engine_shard.cc at lines 784-785, 862-864, and 959-964, then reproduce the tenant-namespace scenario with a blocked XREADGROUP and PEXPIRE. Trace the expiry scan and wake dispatch for non-default namespaces. Done means expired tenant keys are deleted, keyspace notifications and blocked-client wakeups occur, and the reader receives NOGROUP without a later tenant command.

Written by the indexing model from the issue text.