fix: remove PruneOrphanedMessages to preserve history within MAX_HISTORY

PruneOrphanedMessages deleted messages that lost their client_queues
references after PruneOldQueueEntries ran, even when those messages
were within the MAX_HISTORY limit. This made MAX_HISTORY meaningless
for low-traffic channels.

RotateChannelMessages already caps messages per target. Queue pruning
handles client_queues growth. Orphan cleanup is redundant.

Closes sneak/chat#40

README.md

@@ -1789,7 +1789,8 @@ skew issues) and simpler than UUIDs (integer comparison vs. string comparison).
 ### Data Lifecycle
 
 - **Messages**: Rotated per `MAX_HISTORY` — oldest messages beyond the limit
-  are pruned periodically per target (channel or DM).
+  are pruned periodically per target (channel or DM). Orphaned messages (no
+  longer referenced by any client queue) are also removed.
 - **Queue entries**: Pruned automatically when older than `QUEUE_MAX_AGE`
   (default 48h).
 - **Channels**: Deleted when the last member leaves (ephemeral).