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/specs/2026-05-26-ibkr-ingest-design.md

@@ -210,9 +210,12 @@ top.
    - Sections: `Account Information`, `Trades`, `Cash Transactions`,
      `Open Positions`, `Securities Information`
    - Date Format: `yyyy-MM-dd` · Time Format: `HH:mm:ss TimeZone`
-   - Date Range: `Last Business Day` (for daily runs; flip to
-     `Year to Date` only for the initial backfill — irrelevant while
-     account is empty)
+   - Date Range: `Last 90 Days` — trailing window so a missed cron run
+     (failed pod, outage, vacation) doesn't lose data. SyncRecordStore
+     keys on `ibkr:trade:<tradeID>` / `ibkr:cash:<transactionID>`, so
+     overlapping pulls are no-ops. `Last Business Day` was the original
+     choice but creates a "single missed run = permanent data loss"
+     failure mode — rejected in favour of dedup-backed resync window.
    - Format: XML
    - Trade fields: ensure `tradeID`, `tradeDate`, `tradeTime`, `symbol`,
      `buySell`, `quantity`, `tradePrice`, `currency`, `ibCommission`,
@@ -248,10 +251,13 @@ curl -sS -b /tmp/wf-jar -X POST "$WF_BASE_URL/api/v1/accounts" \
 
 ### Step 4 — Initial backfill (skip while account is empty)
 
-When the IBKR account first holds positions, switch the Flex query
-Date Range to `Year to Date`, run the CronJob manually once, verify WF
-totals match the broker app, then switch the Flex query back to
-`Last Business Day` for daily incremental.
+When the IBKR account first holds positions, the daily CronJob will
+backfill automatically up to the 90-day trailing window. For older
+history, temporarily switch the Flex query Date Range to
+`Year to Date` (or `Custom Date Range` with a 1-year window), run the
+CronJob manually once, verify WF totals match the broker app, then
+switch the Flex query back to `Last 90 Days` for daily incremental.
+Dedup makes the temporary widening safe — already-synced rows are no-ops.
 
 ### Step 5 — Deploy