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>

models.py

@@ -36,6 +36,7 @@ class Account(BaseModel):
     description: Optional[str] = None
     user_id: Optional[str] = None  # For user-specific accounts
     created_at: datetime
+    is_active: bool = True  # Soft delete flag
 
 
 class CreateAccount(BaseModel):
@@ -322,6 +323,7 @@ class AccountWithPermissions(BaseModel):
     description: Optional[str] = None
     user_id: Optional[str] = None
     created_at: datetime
+    is_active: bool = True  # Soft delete flag
     # Only included when filter_by_user=true
     user_permissions: Optional[list[PermissionType]] = None
     inherited_from: Optional[str] = None  # Parent account ID if inherited