docs(ibkr): change Flex date range from Last Business Day → Last 90 Days

Trailing window backed by SyncRecordStore dedup is strictly better than
a single-day window — a single missed cron run with Last Business Day
loses that day's activity permanently. SyncRecordStore is keyed by
ibkr:trade:<tradeID> / ibkr:cash:<transactionID>, so overlapping pulls
are no-ops.

Caught during the brainstorming review.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/providers/ibkr.md

@@ -39,8 +39,11 @@ a new query named `broker-sync-activity` with:
 
 **Date Format:** `yyyy-MM-dd`. **Time Format:** `HH:mm:ss` (no timezone
 suffix — ibflex 1.1 rejects timezone abbreviations in the time field).
-**Date Range:** `Last Business Day` for daily incremental. Switch to
-`Year to Date` only for one-off backfills.
+**Date Range:** `Last 90 Days` — trailing window so a missed cron run
+doesn't lose data. SyncRecordStore (keyed by `external_id`) makes
+overlapping pulls idempotent. For a one-off historical backfill, widen
+temporarily to `Year to Date` or `Custom Date Range`, run once, then
+switch back.
 
 ## Cash type mapping