Adds Fava/Beancount integration settings

Adds settings to the Castle extension for integration
with a Fava/Beancount accounting system. This enables
all accounting operations to be managed through Fava.
It includes settings for the Fava URL, ledger slug,
and request timeout.

models.py

@@ -121,6 +121,12 @@ class CastleSettings(BaseModel):
     """Settings for the Castle extension"""
 
     castle_wallet_id: Optional[str] = None  # The wallet ID that represents the Castle
+
+    # Fava/Beancount integration - ALL accounting is done via Fava
+    fava_url: str = "http://localhost:3333"  # Base URL of Fava server
+    fava_ledger_slug: str = "castle-accounting"  # Ledger identifier in Fava URL
+    fava_timeout: float = 10.0  # Request timeout in seconds
+
     updated_at: datetime = Field(default_factory=lambda: datetime.now())
 
     @classmethod