fix: security hardening and Safari stability

- Remove hardcoded JWT_SECRET fallback (crash on boot if unset)
- Make LiveKit config optional with 503 guard on token endpoint
- Add REST rate limiting via @fastify/rate-limit (auth 10/15m, messages 5/5s, uploads 10/1m, global 60/1m)
- Add WebSocket token bucket rate limiter (30 burst, 2/sec refill)
- Add DM channel ownership (ownerId) with migration, enforce on add-member
- Require friendship to add users to group DMs
- Add silent 20Hz oscillator to prevent Safari AudioContext suspension
- Move WebSocket heartbeat to Web Worker to bypass Safari background throttling

packages/server/src/db/schema.ts

@@ -68,6 +68,7 @@ export const attachments = sqliteTable('attachments', {
 
 export const dmChannels = sqliteTable('dm_channels', {
   id: text('id').primaryKey(),
+  ownerId: text('owner_id'),
   createdAt: integer('created_at').notNull(),
 });