Pass preview_id and mime_type through all asset creation fast paths

The duplicate-content upload path and hash-based creation paths were
silently dropping preview_id and mime_type. This wires both fields
through _register_existing_asset, create_from_hash, and all route
call sites so behavior is consistent regardless of whether the asset
content already exists.

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