feat(stats): voice-time and message leaderboards per space

Voice stays get their own table rather than joining the audit log: that table
records points in time, a call is an interval, and pairing join/leave point
events would leave every query guessing at joins whose leave never arrived.

Sessions are opened and closed inside joinRoom/leaveCurrentRoom rather than at
the seven call sites that reach them, so no path can be missed, and
destroyRoom closes them too — it bypasses leaveCurrentRoom and would otherwise
leak open rows.

A restart leaves sessions open with an unknowable end time. They are closed at
startedAt, discarding that time rather than inventing it: crediting the gap
would hand someone hours they never spent, and the numbers are the point.
Mirrors the existing users.status sweep on boot.

Only closed sessions count, so a figure does not move on every refresh. Bars
scale to the leader, not the total — with five people every share of a total
looks identical. Statistics are readable by any member, since they are the
group's own numbers; the audit log, which names who did what, stays on
MANAGE_SPACE.

packages/server/drizzle/meta/_journal.json

@@ -99,6 +99,13 @@
       "when": 1788193162813,
       "tag": "0013_fancy_betty_brant",
       "breakpoints": true
+    },
+    {
+      "idx": 14,
+      "version": "6",
+      "when": 1788193659704,
+      "tag": "0014_mean_killer_shrike",
+      "breakpoints": true
     }
   ]
 }