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/migrate.ts

@@ -117,6 +117,12 @@ export function runMigrations(db: Database.Database): void {
       columns: [
         { name: 'category_id', type: 'TEXT' },
       ]
+    },
+    {
+      name: 'users',
+      columns: [
+        { name: 'profile_updated_at', type: 'INTEGER' },
+      ]
     }
   ];
 
@@ -233,6 +239,9 @@ export function runMigrations(db: Database.Database): void {
   // ─── Convert video channels to voice (video type removed) ─────────────────
   migrateVideoChannels(db);
 
+  // ─── Backfill profile_updated_at from created_at ──────────────────────────
+  migrateProfileUpdatedAt(db);
+
   // ─── Ensure user_space_layout table exists ────────────────────────────────
   db.exec(`
     CREATE TABLE IF NOT EXISTS user_space_layout (
@@ -651,6 +660,16 @@ function migrateVideoChannels(db: Database.Database): void {
   }
 }
 
+/** Backfill profile_updated_at from created_at for existing users */
+function migrateProfileUpdatedAt(db: Database.Database): void {
+  const result = db.prepare(
+    'UPDATE users SET profile_updated_at = created_at WHERE profile_updated_at IS NULL'
+  ).run();
+  if (result.changes > 0) {
+    console.log(`Migrating: Backfilled profile_updated_at for ${result.changes} user(s)`);
+  }
+}
+
 function migrateReplicatedUsernames(db: Database.Database): void {
   const rows = db.prepare(
     "SELECT id, username, home_instance FROM users WHERE home_instance IS NOT NULL AND username NOT LIKE '%@%'"