The previous implementation only did INSERT without handling conflicts.
Now uses GORM's OnConflict clause to properly update existing records
when (model_name, channel_id) unique constraint is violated.

On conflict updates: quota_type, model_ratio, completion_ratio,
model_price, tag_ids, updated_time

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Add new model to support per-channel pricing for models,
allowing the same model to have different prices across
different channels.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>