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.

[nihar-ux18]

Hi @amaydixit11 ! I'm interested in working on this feature.

Before I start, I wanted to clarify a couple of implementation details:

1. For member roles (admin, moderator, member), do you prefer storing the role directly (e.g., as an enum) or using a separate membership model/table that stores the user's role and status within an organization?
2. Should moderators also be able to approve/reject join requests, or should that permission be restricted to admins only?

Also, since this issue includes several features (custom join codes, organization preview, approval workflow, and role-based permissions), would you prefer a single PR or separate PRs for each feature?

Thanks! I'd be happy to work on it once these details are clarified.