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

fava_client.py

@@ -1626,9 +1626,12 @@ class FavaClient:
                             logger.info(f"Account {account_name} already exists in {target_file}")
                             return {"data": sha256sum, "mtime": source_data.get("mtime", "")}
 
-                        # Step 3: Find insertion point (after last Open directive AND its metadata)
+                        # Step 3: Find insertion point (after last Open directive AND its metadata).
+                        # If the file has no Open directives yet (e.g. the empty
+                        # accounts/users.beancount seed), append at end of file
+                        # so the seed header comments stay at the top.
                         lines = source.split('\n')
-                        insert_index = 0
+                        insert_index = None
                         for i, line in enumerate(lines):
                             if line.strip().startswith(('open ', f'{opening_date.year}-')) and 'open' in line:
                                 # Found an Open directive, now skip over any metadata lines
@@ -1636,6 +1639,8 @@ class FavaClient:
                                 # Skip metadata lines (lines starting with whitespace)
                                 while insert_index < len(lines) and lines[insert_index].startswith((' ', '\t')) and lines[insert_index].strip():
                                     insert_index += 1
+                        if insert_index is None:
+                            insert_index = len(lines)
 
                         # Step 4: Format Open directive as Beancount text
                         currencies_str = ", ".join(currencies)
@@ -1989,6 +1994,10 @@ class FavaClient:
 
 # Singleton instance (configured from settings)
 _fava_client: Optional[FavaClient] = None
+# Set by init_fava_client; await for background tasks that must not run
+# before the client exists (otherwise they raise "Fava client not initialized"
+# during the first ~500ms of startup).
+_fava_client_ready: asyncio.Event = asyncio.Event()
 
 
 def init_fava_client(fava_url: str, ledger_slug: str, timeout: float = 10.0):
@@ -2002,9 +2011,21 @@ def init_fava_client(fava_url: str, ledger_slug: str, timeout: float = 10.0):
     """
     global _fava_client
     _fava_client = FavaClient(fava_url, ledger_slug, timeout)
+    _fava_client_ready.set()
     logger.info(f"Fava client initialized: {fava_url}/{ledger_slug}")
 
 
+async def wait_for_fava_client() -> FavaClient:
+    """Block until init_fava_client() has been called, then return the client.
+
+    Use this from background tasks started in libra_start() — they otherwise
+    race the fire-and-forget _init_fava() coroutine and crash with
+    "Fava client not initialized" on first iteration.
+    """
+    await _fava_client_ready.wait()
+    return get_fava_client()
+
+
 def get_fava_client() -> FavaClient:
     """
     Get the configured Fava client.