Assessment PromptEditor: Add video field type #296

Is your feature request related to a problem? Please describe.
The backend now accepts video as an assessment input_schema column type, but the assessment wizard can only mark fields as Text, Image or PDF. There is no way to set up a video assessment from the UI.

Describe the solution you'd like
Add video as a field type in the assessment wizard, alongside image and pdf:

• Offer "Video (link)" in the @ mention type picker.
• Show a video icon for these fields in the mention dropdown, the prompt preview and the Review & save modal.
• Save and load it through the config blob input_schema as { "type": "video", "format": "url" }. Video must be url; base64 returns a 422 when the config is saved.

Backend contract (for reference)

• Cells hold video URLs: http(s):// or gs://. One cell can hold several URLs, separated by commas or newlines. YouTube links work.
• Accepted extensions: .mp4 .mov .mpeg .mpg .avi .wmv .flv .webm .3gp.
• Video is only processed for Google providers (google, google-aistudio, google-gcp). With OpenAI or Anthropic, video cells are skipped silently.

Describe alternatives you've considered
Showing the Video option, or blocking save, only when a Google model is selected. Left out of this change to keep it small, since the backend already validates rows. Can be a follow-up if silent skips turn out to be a problem.