fix(accounts): 409 when admin-adding an account that already exists

add_account no-ops if the Open directive is already present but returned
a normal-looking dict, so the admin endpoint reported success ('created
(sync pending)') for a duplicate. Return an already_existed flag and
raise 409 from the endpoint. Also anchor the existence check on the Open
directive with a trailing-boundary match so a prefix (Expenses:Gas)
doesn't match a longer sibling (Expenses:GasStation). The flag is
additive, so the idempotent user-account path keeps no-opping silently.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

views_api.py

@@ -3695,13 +3695,19 @@ async def api_admin_add_chart_account(
     if payload.description:
         metadata["description"] = payload.description
 
-    await fava.add_account(
+    result = await fava.add_account(
         account_name=payload.name,
         currencies=payload.currencies,
         target_file="accounts/chart.beancount",
         metadata=metadata,
     )
 
+    if result.get("already_existed"):
+        raise HTTPException(
+            status_code=HTTPStatus.CONFLICT,
+            detail=f"Account {payload.name} already exists",
+        )
+
     # Mirror into libra DB so permissions / metadata layer sees it.
     from .account_sync import sync_single_account_from_beancount
     synced = await sync_single_account_from_beancount(payload.name)