feat(P3.1): milestone action endpoints — freeze/start/close + auto-complete hook

- New milestone_actions router with POST freeze/start/close endpoints
- freeze: validates exactly 1 release maintenance task exists
- start: validates all milestone/task dependencies completed, records started_at
- close: allows from open/freeze/undergoing with reason
- try_auto_complete_milestone helper: auto-completes milestone when sole release task finishes
- Wired auto-complete into task transition and update endpoints
- Added freeze enforcement: no new feature story tasks after freeze
- Added started_at to milestone serializer
- All actions write activity logs

app/main.py

@@ -38,6 +38,7 @@ from app.api.routers.monitor import router as monitor_router
 from app.api.routers.milestones import router as milestones_router
 from app.api.routers.roles import router as roles_router
 from app.api.routers.proposes import router as proposes_router
+from app.api.routers.milestone_actions import router as milestone_actions_router
 
 app.include_router(auth_router)
 app.include_router(tasks_router)
@@ -50,6 +51,7 @@ app.include_router(monitor_router)
 app.include_router(milestones_router)
 app.include_router(roles_router)
 app.include_router(proposes_router)
+app.include_router(milestone_actions_router)
 
 
 # Auto schema migration for lightweight deployments