Adds Fava integration for journal entries

Integrates Fava/Beancount for managing journal entries.

This change introduces functions to format entries into Beancount
format and submit them to a Fava instance.

It replaces the previous direct database entry creation with Fava
submission for expense, receivable, and revenue entries. The existing
create_journal_entry function is also updated to submit generic
journal entries to Fava.

beancount_format.py

@@ -462,3 +462,86 @@ def format_payment_entry(
         links=links,
         meta=entry_meta
     )
+
+
+def format_revenue_entry(
+    payment_account: str,
+    revenue_account: str,
+    amount_sats: int,
+    description: str,
+    entry_date: date,
+    fiat_currency: Optional[str] = None,
+    fiat_amount: Optional[Decimal] = None,
+    reference: Optional[str] = None
+) -> Dict[str, Any]:
+    """
+    Format a revenue entry (castle receives payment directly).
+
+    Creates a cleared transaction (flag="*") since payment was received.
+
+    Example: Cash sale, Lightning payment received, bank transfer received.
+
+    Args:
+        payment_account: Payment method account (e.g., "Assets:Bitcoin:Lightning", "Assets:Cash")
+        revenue_account: Revenue account name (e.g., "Income:Sales", "Income:Services")
+        amount_sats: Amount in satoshis (unsigned)
+        description: Entry description
+        entry_date: Date of payment
+        fiat_currency: Optional fiat currency
+        fiat_amount: Optional fiat amount (unsigned)
+        reference: Optional reference
+
+    Returns:
+        Fava API entry dict
+
+    Example:
+        entry = format_revenue_entry(
+            payment_account="Assets:Cash",
+            revenue_account="Income:Sales",
+            amount_sats=100000,
+            description="Product sale",
+            entry_date=date.today(),
+            fiat_currency="EUR",
+            fiat_amount=Decimal("50.00")
+        )
+    """
+    amount_sats_abs = abs(amount_sats)
+    fiat_amount_abs = abs(fiat_amount) if fiat_amount else None
+
+    narration = description
+    if fiat_currency and fiat_amount_abs:
+        narration += f" ({fiat_amount_abs:.2f} {fiat_currency})"
+
+    postings = [
+        format_posting_with_cost(
+            account=payment_account,
+            amount_sats=amount_sats_abs,  # Positive = debit (asset increase)
+            fiat_currency=fiat_currency,
+            fiat_amount=fiat_amount_abs
+        ),
+        format_posting_with_cost(
+            account=revenue_account,
+            amount_sats=-amount_sats_abs,  # Negative = credit (revenue increase)
+            fiat_currency=fiat_currency,
+            fiat_amount=fiat_amount_abs
+        )
+    ]
+
+    entry_meta = {
+        "source": "castle-api",
+        "created-via": "revenue_entry"
+    }
+
+    links = []
+    if reference:
+        links.append(reference)
+
+    return format_transaction(
+        date_val=entry_date,
+        flag="*",  # Cleared (payment received)
+        narration=narration,
+        postings=postings,
+        tags=["revenue-entry"],
+        links=links,
+        meta=entry_meta
+    )