REFACTOR Migrates to single 'amount' field for transactions

Refactors the data model to use a single 'amount' field for journal entry lines, aligning with the Beancount approach.
This simplifies the model, enhances compatibility, and eliminates invalid states.

Includes a database migration to convert existing debit/credit columns to the new 'amount' field.

Updates balance calculation logic to utilize the new amount field for improved accuracy and efficiency.

crud.py

@@ -142,13 +142,13 @@ async def create_journal_entry(
 ) -> JournalEntry:
     entry_id = urlsafe_short_hash()
 
-    # Validate that debits equal credits
-    total_debits = sum(line.debit for line in data.lines)
-    total_credits = sum(line.credit for line in data.lines)
+    # Validate that entry balances (sum of all amounts = 0)
+    # Beancount-style: positive amounts cancel out negative amounts
+    total_amount = sum(line.amount for line in data.lines)
 
-    if total_debits != total_credits:
+    if total_amount != 0:
         raise ValueError(
-            f"Journal entry must balance: debits={total_debits}, credits={total_credits}"
+            f"Journal entry must balance (sum of amounts = 0): sum={total_amount}"
         )
 
     entry_date = data.entry_date or datetime.now()
@@ -191,23 +191,21 @@ async def create_journal_entry(
             id=line_id,
             journal_entry_id=entry_id,
             account_id=line_data.account_id,
-            debit=line_data.debit,
-            credit=line_data.credit,
+            amount=line_data.amount,
             description=line_data.description,
             metadata=line_data.metadata,
         )
         # Insert with metadata as JSON string
         await db.execute(
             """
-            INSERT INTO entry_lines (id, journal_entry_id, account_id, debit, credit, description, metadata)
-            VALUES (:id, :journal_entry_id, :account_id, :debit, :credit, :description, :metadata)
+            INSERT INTO entry_lines (id, journal_entry_id, account_id, amount, description, metadata)
+            VALUES (:id, :journal_entry_id, :account_id, :amount, :description, :metadata)
             """,
             {
                 "id": line.id,
                 "journal_entry_id": line.journal_entry_id,
                 "account_id": line.account_id,
-                "debit": line.debit,
-                "credit": line.credit,
+                "amount": line.amount,
                 "description": line.description,
                 "metadata": json.dumps(line.metadata),
             },
@@ -259,8 +257,7 @@ async def get_entry_lines(journal_entry_id: str) -> list[EntryLine]:
             id=row.id,
             journal_entry_id=row.journal_entry_id,
             account_id=row.account_id,
-            debit=row.debit,
-            credit=row.credit,
+            amount=row.amount,
             description=row.description,
             metadata=metadata,
         )
@@ -364,13 +361,21 @@ async def get_journal_entries_by_user(
 
 
 async def get_account_balance(account_id: str) -> int:
-    """Calculate account balance (debits - credits for assets/expenses, credits - debits for liabilities/equity/revenue)
-    Only includes entries that are cleared (flag='*'), excludes pending/flagged/voided entries."""
+    """
+    Calculate account balance using single amount field (Beancount-style).
+    Only includes entries that are cleared (flag='*'), excludes pending/flagged/voided entries.
+
+    For each account type:
+    - Assets/Expenses: balance = sum of amounts (positive amounts increase, negative decrease)
+    - Liabilities/Equity/Revenue: balance = -sum of amounts (negative amounts increase, positive decrease)
+
+    This works because we store amounts consistently:
+    - Debit (asset/expense increase) = positive amount
+    - Credit (liability/equity/revenue increase) = negative amount
+    """
     result = await db.fetchone(
         """
-        SELECT
-            COALESCE(SUM(el.debit), 0) as total_debit,
-            COALESCE(SUM(el.credit), 0) as total_credit
+        SELECT COALESCE(SUM(el.amount), 0) as total_amount
         FROM entry_lines el
         JOIN journal_entries je ON el.journal_entry_id = je.id
         WHERE el.account_id = :id
@@ -386,13 +391,12 @@ async def get_account_balance(account_id: str) -> int:
     if not account:
         return 0
 
-    total_debit = result["total_debit"]
-    total_credit = result["total_credit"]
+    total_amount = result["total_amount"]
 
     # Use core BalanceCalculator for consistent logic
     core_account_type = CoreAccountType(account.account_type.value)
-    return BalanceCalculator.calculate_account_balance(
-        total_debit, total_credit, core_account_type
+    return BalanceCalculator.calculate_account_balance_from_amount(
+        total_amount, core_account_type
     )
 
 
@@ -500,8 +504,7 @@ async def get_account_transactions(
             id=row.id,
             journal_entry_id=row.journal_entry_id,
             account_id=row.account_id,
-            debit=row.debit,
-            credit=row.credit,
+            amount=row.amount,
             description=row.description,
             metadata=metadata,
         )