fix: temp table schema qualification in pgvector backend

[nicoloesch]

apply_concept_filter_where built IN-subqueries using SQLAlchemy Table objects with no explicit schema. Passing any schema_translate_map to the engine, these references become schema._knn_doms etc. before the query reached PostgreSQL. Temp tables live in pg_temp, so the lookup failed.

Fix: replace the three in_(select(KNN_*_CORE_TABLE.c.id)) calls with text() raw-SQL subqueries, which bypass schema_translate_map and let PostgreSQL resolve the names through the session's implicit pg_temp search-path prefix. The _TEMP_FILTER_METADATA and KNN_*_CORE_TABLE constants are removed as they are no longer used.

The KNN_*_TABLE string constants are kept so setup_concept_filter_temps (DDL) and apply_concept_filter_where (DML) stay in sync by name. SQLite is unaffected, as unqualified names in text() are resolved through SQLite's own temp schema.

• Fixes pgvector backend raises UndefinedTable for temp filter tables when schema_translate_map is active #35

[gkennos]

as discussed - this is a pragmatic hot-fix until temp schema management can be thought through more fully - approving on the assumption that issues will be raise to track that larger concern