Fix incorrect model ID in README Quick Start

[Kamilbenkirane]

Bug

In the README Quick Start section, the videos.analyze example uses model="gemini-3-pro":

video = await celeste.videos.analyze(video_file, prompt="Summarize this clip", model="gemini-3-pro")

gemini-3-pro is not a registered model ID. The correct ID is gemini-3-pro-preview.

Fix

Change to:

video = await celeste.videos.analyze(video_file, prompt="Summarize this clip", model="gemini-3-pro-preview")