fix: batch merge notifications and add storage failure warning

Fixes Copilot audit issues #2 and #4 (code we added):

Issue #2 - Multiple Event Emissions (MEDIUM severity):
- Changed: Collect all merge notifications in array
- Emit: Single batched event instead of multiple separate events
- Impact: Better performance, fewer DB transactions, no UI flickering

Issue #4 - Storage Validation (MEDIUM severity):
- Added: Warning log when LID-PN mappings fail to store
- Tracks: errors count vs notifications sent for debugging
- Improves: Observability of partial storage failures

Technical changes:
- Declared mergeNotifications array before loop
- Compute mergedAt timestamp once (not per iteration)
- Push notifications to array instead of emitting in loop
- Emit single chats.update with all notifications
- Log warning with detailed counts if result.errors > 0

Benefits:
✅ 100x fewer events for 100 mappings (1 vs 100)
✅ Better consumer performance (ZPRO)
✅ Improved observability of storage failures
✅ Zero breaking changes (backward compatible)

Note: Did NOT fix Issue #3 (Prototype Pollution) as it's in
pre-existing code (event-buffer.ts), not code we added.

https://claude.ai/code/session_01SoNUGBEWbJwWWws3F2fuzh

src/Socket/chats.ts

@@ -1309,6 +1309,10 @@ export const makeChatsSocket = (config: SocketConfig) => {
 
 			// Automatic chat merge: notify consumers about LID→PN mapping
 			// This allows ZPRO and other consumers to merge/rename chats accordingly
+			// Collect all merge notifications to emit in a single batch
+			const mergeNotifications: ChatUpdate[] = []
+			const mergedAt = Date.now()
+
 			for (const mapping of mappings) {
 				const lidUser = jidNormalizedUser(mapping.lid)
 				const pnUser = jidNormalizedUser(mapping.pn)
@@ -1316,20 +1320,34 @@ export const makeChatsSocket = (config: SocketConfig) => {
 				if (lidUser && pnUser && lidUser !== pnUser) {
 					logger.debug(
 						{ lid: lidUser, pn: pnUser },
-						'emitting chat update for LID→PN merge notification'
+						'collected chat update for LID→PN merge notification'
 					)
 
-					// Emit chat update indicating this chat should be merged/renamed from LID to PN
-					ev.emit('chats.update', [
-						{
-							id: pnUser,
-							merged: true,
-							previousId: lidUser,
-							mergedAt: Date.now()
-						}
-					])
+					mergeNotifications.push({
+						id: pnUser,
+						merged: true,
+						previousId: lidUser,
+						mergedAt
+					})
 				}
 			}
+
+			// Emit single batch of merge notifications for better performance
+			if (mergeNotifications.length > 0) {
+				logger.debug(
+					{ count: mergeNotifications.length },
+					'emitting batch of chat merge notifications'
+				)
+				ev.emit('chats.update', mergeNotifications)
+			}
+
+			// Log warning if some mappings failed to store
+			if (result.errors > 0) {
+				logger.warn(
+					{ errors: result.errors, total: mappings.length, notified: mergeNotifications.length },
+					'some LID-PN mappings failed to store, but merge notifications were sent'
+				)
+			}
 		} catch (error) {
 			logger.warn({ count: mappings.length, error }, 'Failed to store LID-PN mappings')
 		}