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.

core/validation.py

@@ -23,13 +23,13 @@ def validate_journal_entry(
     entry_lines: List[Dict[str, Any]]
 ) -> None:
     """
-    Validate a journal entry and its lines.
+    Validate a journal entry and its lines (Beancount-style with single amount field).
 
     Checks:
     1. Entry must have at least 2 lines (double-entry requirement)
-    2. Entry must be balanced (sum of debits = sum of credits)
-    3. All lines must have valid amounts (non-negative)
-    4. All lines must have account_id
+    2. Entry must be balanced (sum of amounts = 0)
+    3. All lines must have account_id
+    4. No line should have amount = 0 (would serve no purpose)
 
     Args:
         entry: Journal entry dict with keys:
@@ -38,8 +38,7 @@ def validate_journal_entry(
             - entry_date: datetime
         entry_lines: List of entry line dicts with keys:
             - account_id: str
-            - debit: int
-            - credit: int
+            - amount: int (positive = debit, negative = credit)
 
     Raises:
         ValidationError: If validation fails
@@ -66,64 +65,30 @@ def validate_journal_entry(
                 }
             )
 
-        # Check amounts are non-negative
-        debit = line.get("debit", 0)
-        credit = line.get("credit", 0)
+        # Get amount (Beancount-style: positive = debit, negative = credit)
+        amount = line.get("amount", 0)
 
-        if debit < 0:
+        # Check that amount is non-zero (zero amounts serve no purpose)
+        if amount == 0:
             raise ValidationError(
-                f"Entry line {i + 1} has negative debit: {debit}",
-                {
-                    "entry_id": entry.get("id"),
-                    "line_index": i,
-                    "debit": debit,
-                }
-            )
-
-        if credit < 0:
-            raise ValidationError(
-                f"Entry line {i + 1} has negative credit: {credit}",
-                {
-                    "entry_id": entry.get("id"),
-                    "line_index": i,
-                    "credit": credit,
-                }
-            )
-
-        # Check that a line doesn't have both debit and credit
-        if debit > 0 and credit > 0:
-            raise ValidationError(
-                f"Entry line {i + 1} has both debit and credit",
-                {
-                    "entry_id": entry.get("id"),
-                    "line_index": i,
-                    "debit": debit,
-                    "credit": credit,
-                }
-            )
-
-        # Check that a line has at least one non-zero amount
-        if debit == 0 and credit == 0:
-            raise ValidationError(
-                f"Entry line {i + 1} has both debit and credit as zero",
+                f"Entry line {i + 1} has amount = 0 (serves no purpose)",
                 {
                     "entry_id": entry.get("id"),
                     "line_index": i,
                 }
             )
 
-    # Check entry is balanced
-    total_debits = sum(line.get("debit", 0) for line in entry_lines)
-    total_credits = sum(line.get("credit", 0) for line in entry_lines)
+    # Check entry is balanced (sum of amounts must equal 0)
+    # Beancount-style: positive amounts cancel out negative amounts
+    total_amount = sum(line.get("amount", 0) for line in entry_lines)
 
-    if total_debits != total_credits:
+    if total_amount != 0:
         raise ValidationError(
-            "Journal entry is not balanced",
+            "Journal entry is not balanced (sum of amounts must equal 0)",
             {
                 "entry_id": entry.get("id"),
-                "total_debits": total_debits,
-                "total_credits": total_credits,
-                "difference": total_debits - total_credits,
+                "total_amount": total_amount,
+                "line_count": len(entry_lines),
             }
         )