Add soft delete support for accounts (is_active field)

- Migration m002: Add is_active column to castle_accounts table
- Updated Account and AccountWithPermissions models with is_active field
- Default value: TRUE (all existing accounts remain active)
- Index added for performance on is_active queries

Next steps (to be completed):
- Update account sync to mark orphaned accounts as inactive
- Filter inactive accounts in get_all_accounts queries
- Prevent permissions from being granted on inactive accounts
- Add API endpoint to list/reactivate orphaned accounts

This implements soft delete strategy where accounts removed from
Beancount are marked inactive rather than deleted, preserving
historical data and permissions while preventing new activity.

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

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

migrations.py

@@ -325,3 +325,30 @@ async def m001_initial(db):
                 "description": description
             }
         )
+
+
+async def m002_add_account_is_active(db):
+    """
+    Add is_active field to accounts table for soft delete functionality.
+
+    This enables marking accounts as inactive when they're removed from Beancount
+    while preserving historical data and permissions. Inactive accounts:
+    - Cannot have new permissions granted
+    - Are filtered out of default queries
+    - Can be reactivated if account is re-added to Beancount
+
+    Default: All existing accounts are marked as active (TRUE).
+    """
+    await db.execute(
+        """
+        ALTER TABLE castle_accounts
+        ADD COLUMN is_active BOOLEAN NOT NULL DEFAULT TRUE
+        """
+    )
+
+    # Create index for faster queries filtering by is_active
+    await db.execute(
+        """
+        CREATE INDEX idx_castle_accounts_is_active ON castle_accounts (is_active)
+        """
+    )