Adds background task for invoice processing

Implements a background task that listens for paid invoices
and automatically records them in the accounting system. This
ensures payments are captured even if the user closes their
browser before the client-side polling detects the payment.

Introduces a new `get_journal_entry_by_reference` function to
improve idempotency when recording payments.

crud.py

@@ -226,6 +226,20 @@ async def get_journal_entry(entry_id: str) -> Optional[JournalEntry]:
     return entry
 
 
+async def get_journal_entry_by_reference(reference: str) -> Optional[JournalEntry]:
+    """Get a journal entry by its reference field (e.g., payment_hash)"""
+    entry = await db.fetchone(
+        "SELECT * FROM journal_entries WHERE reference = :reference",
+        {"reference": reference},
+        JournalEntry,
+    )
+
+    if entry:
+        entry.lines = await get_entry_lines(entry.id)
+
+    return entry
+
+
 async def get_entry_lines(journal_entry_id: str) -> list[EntryLine]:
     rows = await db.fetchall(
         "SELECT * FROM entry_lines WHERE journal_entry_id = :id",