Custom Video Backgrounds

Let users upload their own virtual background images (the call already applies starter presets via @livekit/track-processors).

Why deferred

• Per-user blob storage cost + upload/moderation surface for a cosmetic feature (same cost family as custom emoji).
• The preset mechanism already proves the processor pipeline; uploading is only storage + picker work.

Revisit when: users ask for it, or custom emoji ships (the two share the upload/moderation answer).