refactor: replace MandateHistory with Appointment model

[hasnain-saeed]

Replaces the MandateHistory model with an Appointment model that serves as a through table between Member and Position.

Changes:

• Remove MandateHistory model and mandate_history field from Position
• Add Appointment model with:
  • member and position (ForeignKeys)
  • appointed_by (nullable ForeignKey to Member)
  • appointed_date, status, resignation_date, resignation_reason, notes
• Add ManyToManyField relationship between Member and Position using Appointment as the through table

[Copilot]

Pull request overview

This PR refactors the mandate history tracking system by replacing the MandateHistory model with a more feature-rich Appointment model that serves as a through table for a many-to-many relationship between Member and Position.

Key Changes

• Introduces the Appointment model with appointment tracking, status management, and resignation handling
• Removes the MandateHistory model and replaces it with a proper many-to-many relationship using Appointment as the through table
• Adds status tracking for appointments (appointed, resigned, terminated) and related metadata fields

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
backend/backend/models.py	Removed MandateHistory model; added Appointment model with member/position relationships, status tracking, and resignation fields; added positions ManyToManyField to Member; removed mandate_history field from Position; includes code formatting improvements throughout
backend/backend/migrations/0002_appointment_remove_position_mandate_history_and_more.py	Django migration that creates the Appointment model, removes the mandate_history field from Position, deletes the MandateHistory model, and establishes the new many-to-many relationship between Member and Position

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