[P2] Event images - built-in posters and custom upload

[KalebCole]

Priority: P2 (Medium)

Set event cover image - either from Partiful's library or custom upload.

Partiful Built-in Images

Partiful has a poster library with tags. Current default: party-time-badge

# Search Partiful's poster library
partiful posters search "birthday"
partiful posters search "game night"

# Use a specific poster
partiful create --title "Birthday" --poster "birthday-cake-poster"

# Use natural language to find one
partiful create --title "Beach Day" --poster-search "beach summer"

Custom Images

# Upload custom image
partiful create --title "Event" --image ./flyer.png

# Use image URL
partiful create --title "Event" --image "https://example.com/image.png"

Implementation

1. For built-in: query poster library, let user select or fuzzy match
2. For custom: upload to Partiful's storage, get URL, set in event

API Notes

From captured payload, image field structure:

{
  "image": {
    "source": "partiful_posters",
    "poster": { "id": "party-time-badge", "name": "party-time-badge", ... },
    "url": "https://assets.getpartiful.com/posters/party-time-badge"
  }
}

Acceptance Criteria

• List/search Partiful poster library
• Set poster by ID or search term
• Upload custom image
• Show image preview URL after creation