Add settlement links to payment entries for traceability

- Add settled_entry_links parameter to format_payment_entry and format_net_settlement_entry
- Query unsettled expenses/receivables before creating settlement entries
- Pass original entry links to format functions so settlements reference what they settle
- Update all callers in views_api.py (5 locations) and tasks.py (1 location)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tasks.py

@@ -287,6 +287,18 @@ async def on_invoice_paid(payment: Payment) -> None:
             logger.error("Lightning account 'Assets:Bitcoin:Lightning' not found")
             return
 
+        # Query for unsettled entries to link this settlement back to them
+        # Net settlement can settle both expenses and receivables
+        settled_links = []
+        try:
+            unsettled_expenses = await fava.get_unsettled_entries_bql(user_id, "expense")
+            settled_links.extend([e["link"] for e in unsettled_expenses if e.get("link")])
+            unsettled_receivables = await fava.get_unsettled_entries_bql(user_id, "receivable")
+            settled_links.extend([e["link"] for e in unsettled_receivables if e.get("link")])
+        except Exception as e:
+            logger.warning(f"Could not query unsettled entries for settlement links: {e}")
+            # Continue without links - settlement will still be recorded
+
         # Format as net settlement transaction
         entry = format_net_settlement_entry(
             user_id=user_id,
@@ -301,7 +313,8 @@ async def on_invoice_paid(payment: Payment) -> None:
             description=f"Lightning payment settlement from user {user_id[:8]}",
             entry_date=datetime.now().date(),
             payment_hash=payment.payment_hash,
-            reference=payment.payment_hash
+            reference=payment.payment_hash,
+            settled_entry_links=settled_links if settled_links else None
         )
 
         # Submit to Fava