## Critical Implementation Revisions  

### Phase 1.1: Core Model Updates (2 Days)  
1. Add lock fields to VersionBranch  
2. Implement StateMachine base class  
3. Extend HistoricalChangeMixin with structured diffs  

### Phase 2.1: Manager Classes (3 Days)  
```python  
class LockManager(models.Manager):  
    def get_locked_branches(self):  
        return self.filter(lock_status__isnull=False)  

class StateMachine:  
    def __init__(self, workflow):  
        self.states = workflow['states']  
        self.transitions = workflow['transitions']  
```  

### Phase 3.1: Security Backports (1 Day)  
- Add model clean() validation  
- Implement permission check decorators