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/server/src/db/schema.ts

@@ -180,6 +180,7 @@ export const serverFolderMembers = sqliteTable('server_folder_members', {
 export const instanceSettings = sqliteTable('instance_settings', {
   id: integer('id').primaryKey().default(1),
   instanceName: text('instance_name').default('Backspace'),
+  workerId: integer('worker_id'),
   maxBitrateKbps: integer('max_bitrate_kbps').notNull().default(20000),
   minBitrateKbps: integer('min_bitrate_kbps').notNull().default(500),
   bitrateStepKbps: integer('bitrate_step_kbps').notNull().default(500),