Creates accounts in Fava if they don't exist

This change ensures that user-specific accounts are automatically created
in the Fava/Beancount ledger when they are first requested. It checks for
the existence of the account via a Fava query and creates it via an Open
directive if it's missing.  This simplifies account management and
ensures that all necessary accounts are available for transactions.

This implementation adds a new `add_account` method to the `FavaClient`
class which makes use of the /add_entries endpoint to create an account
using an Open Directive.

fava_client.py

@@ -6,9 +6,11 @@ All accounting logic is delegated to Fava/Beancount.
 
 Fava provides a REST API for:
 - Adding transactions (PUT /api/add_entries)
+- Adding accounts via Open directives (PUT /api/add_entries)
 - Querying balances (GET /api/query)
 - Balance sheets (GET /api/balance_sheet)
 - Account reports (GET /api/account_report)
+- Updating/deleting entries (PUT/DELETE /api/source_slice)
 
 See: https://github.com/beancount/fava/blob/main/src/fava/json_api.py
 """
@@ -679,6 +681,73 @@ class FavaClient:
             logger.error(f"Fava connection error: {e}")
             raise
 
+    async def add_account(
+        self,
+        account_name: str,
+        currencies: list[str],
+        opening_date: Optional[date] = None,
+        metadata: Optional[Dict[str, Any]] = None
+    ) -> Dict[str, Any]:
+        """
+        Add an account to the Beancount ledger via an Open directive.
+
+        Args:
+            account_name: Full account name (e.g., "Assets:Receivable:User-abc123")
+            currencies: List of currencies for this account (e.g., ["EUR", "SATS"])
+            opening_date: Date to open the account (defaults to today)
+            metadata: Optional metadata for the account
+
+        Returns:
+            Response from Fava ({"data": "Stored 1 entries.", "mtime": "..."})
+
+        Example:
+            # Add a user's receivable account
+            result = await fava.add_account(
+                account_name="Assets:Receivable:User-abc123",
+                currencies=["EUR", "SATS", "USD"],
+                metadata={"user_id": "abc123", "description": "User receivables"}
+            )
+
+            # Add a user's payable account
+            result = await fava.add_account(
+                account_name="Liabilities:Payable:User-abc123",
+                currencies=["EUR", "SATS"]
+            )
+        """
+        from datetime import date as date_type
+
+        if opening_date is None:
+            opening_date = date_type.today()
+
+        # Format Open directive for Fava
+        open_directive = {
+            "t": "Open",
+            "date": opening_date.isoformat(),
+            "account": account_name,
+            "currencies": currencies,
+            "meta": metadata or {}
+        }
+
+        try:
+            async with httpx.AsyncClient(timeout=self.timeout) as client:
+                response = await client.put(
+                    f"{self.base_url}/add_entries",
+                    json={"entries": [open_directive]},
+                    headers={"Content-Type": "application/json"}
+                )
+                response.raise_for_status()
+                result = response.json()
+
+                logger.info(f"Added account {account_name} to Fava with currencies {currencies}")
+                return result
+
+        except httpx.HTTPStatusError as e:
+            logger.error(f"Fava HTTP error adding account: {e.response.status_code} - {e.response.text}")
+            raise
+        except httpx.RequestError as e:
+            logger.error(f"Fava connection error: {e}")
+            raise
+
 
 # Singleton instance (configured from settings)
 _fava_client: Optional[FavaClient] = None