feat(cache): update database layer for expanded metadata

- Update upsert_file() to include all 26 new AudioMeta fields
- Update get_file_by_virtual_path() to read all new columns
- Add get_file_metadata_row() for overlay synthesis
- Add update_metadata() for partial metadata updates
- Add clear_overlay() to reset metadata to NULL
- Handle format_layout BLOB with msgpack serialization
- Handle custom_tags JSON with serde_json
- Add 8 comprehensive unit tests
- All 92 tests pass, LSP diagnostics clean

crates/musicfs-cache/Cargo.toml

@@ -14,6 +14,7 @@ tracing.workspace = true
 thiserror.workspace = true
 serde.workspace = true
 rmp-serde.workspace = true
+serde_json.workspace = true
 image.workspace = true
 lofty = "0.24"
 parking_lot.workspace = true