CSO-11: member status management system

[shanoysinc]

Description ✏️

Closes #11

This PR implements a member status management system with status-based access control and external integrations.

✨ Features & Improvements

• Added member status enum: active, inactive, bulk_removed, banned
• Implemented batch member status updates
• Added status update event handler to sync Airtable, Mailchimp, and Slack
• Added individual member status update page in the admin dashboard
• Updated onboarding completion to automatically activate members and re-activate Slack/Mailchimp
• Updated student removal flow to use status updates instead of deletion
• Prevented inactive / bulk_removed / banned members from signing in
• Prevented auto-rejection of applications for bulk-removed students
• Added database migration for the member status column

🐛 Fixes

• Fixed duplicate pending application issue
• Fixed incorrect error message for deactivated members
• Ensured Airtable status updates sync properly

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Type of Change 🐞

• Feature — A non-breaking change that adds functionality
• Fix — A non-breaking change that fixes an issue

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Checklist ✅

• I have done a self-review of my code
• I have manually tested my code (if applicable)