Per-video unique key (video_key) now available across analytics

Every analytics event now carries a stable video_key that uniquely identifies a piece of content within a workspace. The key is generated at ingestion time, persisted alongside the event in the Data pipeline, and synced into the workspace data layer so it can be referenced from other FastPix services. What this means for customers:

1. Stable grouping — different beacons for the same video (across players, sessions, and devices) now roll up under a single key, even when the underlying title or source URL changes.
2. Available as a dimension — video_key can be used as a filter or breakdown dimension in queries.
3. Resilient lookup — the key resolves correctly even on cold starts, thanks to layered caching in the ingestion path.