Add generic Gathering system for flexible event/meetup management

[Copilot]

The platform lacked a standalone, type-agnostic event model. Course Session is tightly coupled to courses, leaving no way to organize meetups, club meetings, workshops, webinars, or any ad-hoc gathering independent of a course.

New Models

• Gathering — core event entity with 10 configurable types (meetup, workshop, class, webinar, conference, study_group, networking, hackathon, club_meeting, other), virtual/in-person toggle, capacity limits, pricing, visibility (public/private/invite_only), and status lifecycle (draft → published → completed/cancelled)
• GatheringRegistration — attendee registration with status workflow (pending → confirmed → attended; waitlisted, cancelled); auto-waitlists when capacity is reached
• GatheringAnnouncement — organizer-posted announcements broadcast to registrants

Views & URLs (/gatherings/)

URL	View
/gatherings/	Filterable public list (type, keyword, virtual/in-person)
/gatherings/create/	Create new gathering
/gatherings/mine/	Organizer dashboard
/gatherings/<slug>/	Detail + registration sidebar
/gatherings/<slug>/edit/	Edit (organizer only)
/gatherings/<slug>/manage/	Attendee status management + post announcements
/gatherings/<slug>/register/	Register / join waitlist
/gatherings/<slug>/cancel-registration/	Cancel registration

Templates

Five Tailwind CSS templates with full dark mode: list, detail, create (shared edit), manage, my_gatherings. Meeting link is gated behind confirmed registration for virtual gatherings.

Other

• Django admin registration for all three models with list_display, list_filter, search_fields
• Migration 0064
• 25 model + view tests covering slug uniqueness, capacity/waitlist logic, visibility enforcement, organizer-only access, and CRUD flows

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