Adds fiat currency support for expenses

Extends expense entry functionality to support fiat currencies.

Users can now specify a currency (e.g., EUR, USD) when creating expense entries. The specified amount is converted to satoshis using exchange rates. The converted amount and currency information are stored in the journal entry metadata. Also adds an API endpoint to retrieve allowed currencies and updates the UI to allow currency selection when creating expense entries.

migrations.py

@@ -61,7 +61,8 @@ async def m001_initial(db):
             account_id TEXT NOT NULL,
             debit INTEGER NOT NULL DEFAULT 0,
             credit INTEGER NOT NULL DEFAULT 0,
-            description TEXT
+            description TEXT,
+            metadata TEXT DEFAULT '{{}}'
         );
         """
     )