ibkr: use IBKR account number as the canonical Account.id

Bug: provider passed the WF UUID as Account.id. ensure_account looks up
existing accounts by (provider, providerAccountId=Account.id), so the
WF-UUID-as-providerAccountId would never match the manually-created
account (which has providerAccountId=U13279690), causing the pipeline
to create a duplicate WF account on every cron run.

Fix: Account.id is now the IBKR account number (U13279690) throughout.
The pipeline's _ensure_accounts() resolves it to the WF UUID via the
canonical (provider, providerAccountId) lookup; activities are remapped
before import. CLI no longer takes the WF UUID — derives it post-import
via a cheap idempotent ensure_account call for the reconciliation step.

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

broker_sync/providers/ibkr.py

@@ -179,12 +179,14 @@ class IBKRProvider:
         *,
         token: str,
         query_id: str,
-        wf_account_id: str,
         upstream_account_id: str,
     ) -> None:
         self._token = token
         self._query_id = query_id
-        self._wf_account_id = wf_account_id
+        # Single source of truth — the IBKR account number (e.g. U13279690).
+        # The pipeline's _ensure_accounts() resolves this to a Wealthfolio
+        # UUID via (provider="ibkr", providerAccountId=upstream_account_id);
+        # activities are remapped to the WF UUID before import.
         self._upstream_account_id = upstream_account_id
         # Stashed for the reconciliation step after fetch() drains.
         self._last_response: Any = None
@@ -192,7 +194,7 @@ class IBKRProvider:
     def accounts(self) -> list[Account]:
         return [
             Account(
-                id=self._wf_account_id,
+                id=self._upstream_account_id,
                 name="Interactive Brokers (UK)",
                 account_type=AccountType.GIA,
                 currency="GBP",  # FX-aware per-trade; account ccy is GBP
@@ -232,10 +234,10 @@ class IBKRProvider:
             )
 
         for trade in stmt.Trades or []:
-            yield _map_trade_to_activity(trade, account_id=self._wf_account_id)
+            yield _map_trade_to_activity(trade, account_id=self._upstream_account_id)
 
         for cash in stmt.CashTransactions or []:
-            activity = _map_cash_to_activity(cash, account_id=self._wf_account_id)
+            activity = _map_cash_to_activity(cash, account_id=self._upstream_account_id)
             if activity is not None:
                 yield activity