fix(sync): raise page limit from 50 to 5000 for large chat histories

50 pages (50k messages) was too low for groups with 100k+ messages.
Raise to 5000 pages (5M messages) as a safety valve while allowing
full-sync to complete in a single pull cycle for most conversations.

packages/sync/src/pull-engine.ts

@@ -22,7 +22,7 @@ import type {
 } from './types'
 import type { DataSourceManager } from './data-source-manager'
 
-const MAX_PAGES_PER_PULL = 50
+const MAX_PAGES_PER_PULL = 5000
 const PULL_OVERLAP_SECONDS = 60
 
 // ==================== Helpers ====================