Add interest events run by interest group leaders

[ch0rizo]

Description

Apart of a stack. Wanted to try github stacked PRs (its very nice).

Adds the interest_event type: leaders and co-leaders of active interest groups can create
and manage events for their own group, without keyword permissions.

• Locked field contract in constants.py - creators control content fields; INFINITE status,
  free, no captcha/penalties/feedback, and never pinned are forced by the serializer
• One backend-owned pool, open to all of Abakus from creation until start, with optional
  capacity and waiting list
• Registration/unregistration run synchronously with the async pipeline as fallback, so the
  response carries the final result
• Current group leaders can always edit/delete the group's events; administrate stays
  keyword-gated for interest events (creators of other event types keep object access)
• Leadership upkeep: the last leader cannot demote themselves; leaving auto-promotes the
  oldest co-leader or deactivates the group (also reconciled nightly)
• Interest events are excluded from the frontpage and general ical feeds; the events list
  gains exclude_event_type and responsible_group_type filters

Testing

• The code quality is at a minimum required level of quality, readability, and performance.
• I have thoroughly tested my changes.

Please describe what and how the changes have been tested, and provide instructions to reproduce if necessary.

Tested with tox -e tests. Frontend PR in lego-webapp follows and needs this deployed first.

Resolves #4180

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}