feat: Add custom join codes and admin approval workflow for organizations

[amaydixit11]

Feature Request

Problem

The organization system uses randomly generated join codes (models.py, line 246: uuid.uuid4().hex[:10].upper()) which are impossible to remember. There's no way to visually verify that you're joining the right organization.

Proposed Solution

1. Allow custom alphanumeric join codes like "IITBH-DSA-2025" during organization creation
2. Show organization card (name, description, member count) before joining by join code
3. Add an "Approve/Reject" workflow for join requests (admin approval required)
4. Role-based member tiers: admin, moderator, member with different permissions

Technical Notes

The Organization.save() method already generates join_codes. Custom codes would need a uniqueness check and format validation. The existing /api/organizations/join endpoint could show org details before confirming the join.