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.

migrations.py

@@ -541,3 +541,90 @@ async def m014_remove_legacy_equity_accounts(db):
         "DELETE FROM accounts WHERE name = :name",
         {"name": "Equity:RetainedEarnings"}
     )
+
+
+async def m015_convert_to_single_amount_field(db):
+    """
+    Convert entry_lines from separate debit/credit columns to single amount field.
+
+    This aligns Castle with Beancount's elegant design:
+    - Positive amount = debit (increase assets/expenses, decrease liabilities/equity/revenue)
+    - Negative amount = credit (decrease assets/expenses, increase liabilities/equity/revenue)
+
+    Benefits:
+    - Simpler model (one field instead of two)
+    - Direct compatibility with Beancount import/export
+    - Eliminates invalid states (both debit and credit non-zero)
+    - More intuitive for programmers (positive/negative instead of accounting conventions)
+
+    Migration formula: amount = debit - credit
+
+    Examples:
+    - Expense transaction:
+      * Expenses:Food:Groceries   amount=+100 (debit)
+      * Liabilities:Payable:User  amount=-100 (credit)
+    - Payment transaction:
+      * Liabilities:Payable:User     amount=+100 (debit)
+      * Assets:Bitcoin:Lightning     amount=-100 (credit)
+    """
+    from sqlalchemy.exc import OperationalError
+
+    # Step 1: Add new amount column (nullable for migration)
+    try:
+        await db.execute(
+            "ALTER TABLE entry_lines ADD COLUMN amount INTEGER"
+        )
+    except OperationalError:
+        # Column might already exist if migration was partially run
+        pass
+
+    # Step 2: Populate amount from existing debit/credit
+    # Formula: amount = debit - credit
+    await db.execute(
+        """
+        UPDATE entry_lines
+        SET amount = debit - credit
+        WHERE amount IS NULL
+        """
+    )
+
+    # Step 3: Create new table with amount field as NOT NULL
+    # SQLite doesn't support ALTER COLUMN, so we need to recreate the table
+    await db.execute(
+        """
+        CREATE TABLE entry_lines_new (
+            id TEXT PRIMARY KEY,
+            journal_entry_id TEXT NOT NULL,
+            account_id TEXT NOT NULL,
+            amount INTEGER NOT NULL,
+            description TEXT,
+            metadata TEXT DEFAULT '{}'
+        )
+        """
+    )
+
+    # Step 4: Copy data from old table to new
+    await db.execute(
+        """
+        INSERT INTO entry_lines_new (id, journal_entry_id, account_id, amount, description, metadata)
+        SELECT id, journal_entry_id, account_id, amount, description, metadata
+        FROM entry_lines
+        """
+    )
+
+    # Step 5: Drop old table and rename new one
+    await db.execute("DROP TABLE entry_lines")
+    await db.execute("ALTER TABLE entry_lines_new RENAME TO entry_lines")
+
+    # Step 6: Recreate indexes
+    await db.execute(
+        """
+        CREATE INDEX idx_entry_lines_journal_entry ON entry_lines (journal_entry_id)
+        """
+    )
+
+    await db.execute(
+        """
+        CREATE INDEX idx_entry_lines_account ON entry_lines (account_id)
+        """
+    )