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/src/db/schema.ts

@@ -610,3 +610,26 @@ export const auditEvents = sqliteTable('audit_events', {
   spaceIdx: index('idx_audit_events_space_created').on(table.spaceId, table.createdAt),
   actorIdx: index('idx_audit_events_actor').on(table.actorId),
 }));
+
+/**
+ * One row per stay in a voice room, closed when the user leaves.
+ *
+ * Separate from `auditEvents` on purpose: that table records points in time,
+ * while a call is an interval. Storing joins and leaves as separate point
+ * events would make every statistics query pair rows by hand and guess at
+ * joins whose leave never arrived (a crash, a restart).
+ *
+ * `endedAt` null means still connected. `spaceId` is null for DM calls.
+ */
+export const voiceSessions = sqliteTable('voice_sessions', {
+  id: text('id').primaryKey(),
+  spaceId: text('space_id').references(() => spaces.id, { onDelete: 'cascade' }),
+  channelId: text('channel_id').notNull(),
+  userId: text('user_id').notNull().references(() => users.id, { onDelete: 'cascade' }),
+  startedAt: integer('started_at').notNull(),
+  endedAt: integer('ended_at'),
+}, (table) => ({
+  spaceIdx: index('idx_voice_sessions_space_started').on(table.spaceId, table.startedAt),
+  userIdx: index('idx_voice_sessions_user').on(table.userId),
+  openIdx: index('idx_voice_sessions_ended').on(table.endedAt),
+}));