feat: LWW timestamps for bidirectional federation profile & layout sync

Profile and space layout changes on remote instances were being
overwritten by stale data on reconnect. Adds Last-Writer-Wins
timestamps so the client-relay mesh rejects stale writes:

- profile_updated_at column on users table with migration + backfill
- Server LWW guards on PATCH /users/@me and PUT /space-layout
- Bidirectional profileSync: pulls newer remote profiles to home
- LWW layout sync replaces home-authoritative _layoutFromTrueHome flag
- Layout pushes to ALL connected instances in parallel

packages/server/src/db/schema.ts

@@ -17,6 +17,7 @@ export const users = sqliteTable('users', {
   avatarColor: text('avatar_color'),
   bio: text('bio'),
   isDeleted: integer('is_deleted').default(0),
+  profileUpdatedAt: integer('profile_updated_at'),
   createdAt: integer('created_at').notNull(),
 });