feat: add Discovery model and ViewSet with expanded event data

[dragonee]

Summary

• Add Discovery event type with ManyToMany relationship to Events
• Add REST API endpoint at /discoveries/ for managing Discovery items
• Implement full CRUD operations via DiscoveryViewSet
• Add event expansion with complete JSON serialization

Changes

Models & Database

• Add Discovery model extending Event with name, comment, and events fields
• Create migration 0062_discovery.py for new model

API & Serializers

• Add BasicDiscoverySerializer for EventSerializer mapping (prevents circular references)
• Add DiscoverySerializer with expanded event data:
  • events (read-only): Full event JSON via EventSerializer
  • event_ids (write-only): Accept event IDs for creating/updating
• Add DiscoveryEventsRequestSerializer for filtered event selection

Views & URLs

• Create DiscoveryViewSet with full CRUD operations
• Register /discoveries/ API endpoint
• Add /api/events/discovery/ endpoint for filtered event selection

Admin

• Add DiscoveryAdmin with event filter_horizontal interface
• Register Discovery in polymorphic EventAdmin

API Usage

GET /discoveries/{id}/ - Returns Discovery with expanded events:

{
  "id": 1,
  "name": "Important Discovery",
  "comment": "Discovery details",
  "events": [
    {
      "id": 42,
      "comment": "Journal entry",
      "resourcetype": "JournalAdded",
      ...
    }
  ]
}

POST /discoveries/ - Create Discovery:

{
  "name": "New Discovery",
  "comment": "Details",
  "thread": "Daily",
  "event_ids": [42, 87]
}

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