Add confirmation reminder and parameterized Slack invite endpoints

[Copilot]

Adds two admin endpoints to facilitate event communication: one to remind accepted hackers to confirm attendance, and another to send Slack invitations with a configurable URL.

Changes

• New endpoint POST /v1/event/{event_id}/send_confirmation_reminder/

  • Sends reminders only to accepted hackers who haven't confirmed (confirmed_assistance = false)
  • Auto-generates confirmation tokens if missing
  • Returns count of emails queued

• New endpoint POST /v1/event/{event_id}/send_slack_invite/

  • Accepts slack_url in request body instead of hardcoded value
  • Sends to all accepted hackers
  • Returns count of emails queued

• Email template EVENT_CONFIRMATION_REMINDER added to InternalTemplate enum

Usage

# Send confirmation reminders
POST /v1/event/123/send_confirmation_reminder/
Authorization: Bearer <admin_token>
→ {"success": true, "sent": 15}

# Send Slack invites
POST /v1/event/123/send_slack_invite/
Authorization: Bearer <admin_token>
Body: {"slack_url": "https://join.slack.com/t/..."}
→ {"success": true, "sent": 50}

Both require LLEIDAHACKER role. The existing send_slack_mail endpoint remains for backward compatibility.

Original prompt

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This section details on the original issue you should resolve

<issue_title>New enponts</issue_title>
<issue_description>Create two new endpoints. First to remember to the participants to confirm assistence and second endpoint to send slack url</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes New enponts #330

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