Adopt `transitions` library for consultation queue state machine

Problem

Queue status transitions in app/services/consultation_queue_service.py are implemented as a manually maintained VALID_TRANSITIONS dict with imperative validation logic (~120 LOC). Adding a new state or transition requires editing multiple places and is error-prone.

The full flow is: vitals → know_your_patient → waiting → in_consultation → consultation_completed → medicine_prescribed → medicine_verified → counselling_done

Location

app/services/consultation_queue_service.py lines ~283–399

Fix

Replace with the transitions library (LGPL-3.0, actively maintained):

from transitions import Machine

states = [s.value for s in QueueStatusEnum]
transitions_list = [
    {'trigger': 'advance', 'source': 'vitals', 'dest': 'know_your_patient'},
    {'trigger': 'advance', 'source': 'waiting', 'dest': 'in_consultation'},
    # ...
]
machine = Machine(model=queue_obj, states=states, transitions=transitions_list)

Invalid transitions are rejected automatically; no manual validation code needed.

References

• https://github.com/pytransitions/transitions

Effort

Medium (~3h)