fix: persistent random Snowflake worker ID + clean reaction API

Two fixes addressing architectural review feedback:

1. Snowflake ID collisions: Replace process.pid-based worker ID with a
   cryptographically random value (0-1023) generated once at first boot
   and persisted to instance_settings.worker_id. Eliminates deterministic
   ID collisions between Docker instances that all run as PID 1.

2. Reaction API leak: Revert addReaction/removeReaction signatures to
   (messageId, emoji) — the store now resolves the channel internally by
   scanning its message cache, keeping routing logic out of the UI layer.

packages/web/src/components/chat/Message.tsx

@@ -60,9 +60,9 @@ export function Message({ message, isCompact, isFirstInGroup }: MessageProps) {
   const toggleReaction = (emoji: string) => {
     const hasReacted = message.reactions?.some(r => r.userId === currentUser?.id && r.emoji === emoji);
     if (hasReacted) {
-      removeReaction(message.id, emoji, channelKey);
+      removeReaction(message.id, emoji);
     } else {
-      addReaction(message.id, emoji, channelKey);
+      addReaction(message.id, emoji);
     }
   };