Extract rebuild_metadata_projection() to build AssetReferenceMeta rows
from {**system_metadata, **user_metadata}, so system-generated metadata
is queryable via metadata_filter and user keys override system keys.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Prevents cross-tenant metadata mutation when multiple references share
the same content-addressed Asset row. mime_type can now only be set when
NULL (first ingest); subsequent attempts to change it are silently ignored.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Clients receive preview_id in API responses but could not dereference it
through public routes (which use reference IDs). Now preview_id is a
self-referential FK to asset_references.id so the value is directly
usable in the public API.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
