feat: allow for multiple metadata filters on list fields

[MattiaMolon]

Summary

This PR refactors metadata filtering so search and delete consistently support multi-value filters with clear semantics, and updates self-query so it can select multiple filters per field.

What Changed

Metadata filtering implementation

• Added a new shared helper: src/raglite/_metadata_filter.py
  • build_metadata_filter_condition(...) for SQLAlchemy query conditions.
  • build_metadata_filter_sql(...) for raw SQL query fragments + params.
• Updated search paths to use shared metadata filter logic:
  • src/raglite/_search.py
    • vector_search() now uses shared condition builder.
    • keyword_search() now uses shared SQL builder for PostgreSQL and DuckDB.
• Updated delete path to use shared metadata filter logic:
  • src/raglite/_delete.py
    • _get_documents_with_metadata() now builds conditions via shared helper.

Self-query improvements

• Updated src/raglite/_search.py self-query prompt and mapping logic so a single metadata field can return multiple matched values.
• Self-query output now keeps multiple valid values per field (with deduplication and invalid ID filtering).

Tests

• tests/test_search.py
  • Added tests for OR semantics with multi-value filters.
  • Added tests for matching list-valued document metadata.
  • Added _self_query test to verify multiple values per field are preserved.
• tests/test_delete.py
  • Added test for delete-by-metadata with multi-value OR semantics.

Docs

• README.md
  • Documented list metadata storage behavior.
  • Added multi-value metadata filter example.

[MattiaMolon]

Can be merged as far as I can tell @emilradix