Polish account-creation flow: insertion point, user_id consistency, startup race

Three small fixes shaken out by live testing on aio-demo:

1. fava_client.add_account: when the target file has no Open directives
   yet (e.g. the empty accounts/users.beancount seed), append at end of
   file instead of inserting at index 0. Keeps the seed header comments
   at the top where they belong.

2. account_sync.sync_single_account_from_beancount: read the full user_id
   from Beancount metadata when present, fall back to the name-derived
   8-char prefix otherwise. crud.get_or_create_user_account writes the
   full 32-char user_id into Beancount metadata when creating per-user
   accounts; the sync function was only looking at the account name and
   returning the prefix, so the post-sync `WHERE user_id=:user_id` query
   in crud.py missed the row and fell through the UNIQUE-constraint
   recovery path. Three lines of warning noise per user-account creation.

3. tasks.wait_for_account_sync: await `wait_for_fava_client()` (new
   helper backed by an asyncio.Event in fava_client.py) before the first
   sync iteration. Previously the sync task started in libra_start()
   raced the fire-and-forget `_init_fava()` coroutine and reliably
   crashed the first run with "Fava client not initialized".

Refs: aiolabs/libra#28

tasks.py

@@ -134,8 +134,15 @@ async def wait_for_account_sync():
     Background task that periodically syncs accounts from Beancount to Libra DB.
 
     Runs hourly to ensure Libra DB stays in sync with Beancount.
+
+    Blocks on `wait_for_fava_client()` before the first iteration so we don't
+    race the fire-and-forget `_init_fava()` started in `libra_start()` and
+    fail the first sync with "Fava client not initialized".
     """
+    from .fava_client import wait_for_fava_client
+
     logger.info("[LIBRA] Account sync background task started")
+    await wait_for_fava_client()
 
     while True:
         try: