Replaced the Fields() type switch with a fields() method on the
Expression interface, eliminating the need to update a central switch
when adding new expression types. Removed the now-redundant
criteriaExpression() marker method since fields() alone suffices to
restrict the interface. Extracted a conjunction interface for the
ChildPlaylistIds() lookup used by All and Any.
* refactor: move criteria SQL generation to persistence
Keep model/criteria as a domain DSL with JSON parsing, field metadata, expression traversal, and child playlist extraction only. Move smart playlist SQL translation, sort SQL, and join planning into persistence behind smartPlaylistCriteria so repository code uses a small query-building API.
* refactor: simplify criteria translator metadata
Use generic helper functions for criteria operator maps so the SQL translator can pass named criteria map types directly. Remove unused pseudo-field metadata from the criteria field API while preserving special field name lookup.
* test: add coverage check for criteria-to-SQL field mappings
Add a test that iterates all fields registered in the criteria package and
verifies that every non-tag/non-role field has a corresponding entry in
the persistence layer's smartPlaylistFields map. This prevents silent
drift between the domain field registry and the SQL translation layer.
Also adds an AllFieldNames() function to the criteria package to support
field enumeration from outside the package.
