Add DuckDB backend

[nicoloesch]

Summary

• Closes: Feasibility review for addition of duckdb backend #4
• Depends on: Add DuckDB dialect support oa-configurator#44

Changes

• Adds a DuckDB backend through a new duckdb extra, with staging tables and batched replace, upsert and insert merges.
• Backends declare foreign keys they can't enforce through deferred_fk_constraints().
• load_csv(validate_deferred_fks=True) checks those keys with an anti-join after each merge.
• The new check does nothing on PostgreSQL and SQLite, which have no deferred keys.
• On DuckDB, replace updates rows in place, because DuckDB rejects deleting a referenced row.
• check_replace_feasible() lets callers reject an impossible replace before loading anything.
• replace with index_strategy="keep" is rejected up front on backends that can't support it.
• Pagination lives in one shared _paginate_batches helper instead of a copy per backend.
• Each backend now declares whether it commits between batches.
• load_csv always drops its staging table, including when the merge fails.
• Depends on the DuckDB dialect in , so CI fails until that is released.

Checklist

• Applied exactly one label (breaking, feature, fix, dependencies, or chore)
• Tests pass locally (uv run pytest -q)
• Lint passes (uv run ruff check .)