v0.4.0 - Backend-agnostic QueryContext, ClickHouse reference backend

Breaking change for anyone who wrote a custom Filter, Sorter, QueryStrategy, or
PaginationStrategy subclass. Not breaking for the plain README flow (GenericDao +
generic_filters + default strategies) — that continues to work unchanged. See CHANGELOG.md
for the full breakdown and migration table.

Highlights

• Introduces QueryContext as the real seam behind Filter/Sorter/Paginator/QueryStrategy —
  SqlAlchemyQueryContext (default) and a new ClickHouseQueryContext/ClickHouseDao reference
  backend prove the abstraction genuinely generalizes beyond SQLAlchemy.
• import fastapi_listing no longer requires SQLAlchemy to be installed.
• Escape hatches for cases canonical filters shouldn't be forced to express: HavingMixin,
  order_by_raw(), add_raw_condition(), from_raw_sql() — all validated against a real
  ClickHouse server.
• Bug fixes: grouped-count double-counting in ClickHouse, a ContextVar leak in
  middlewares.py, wrong 409 → 422 status code for unregistered filter/sort fields.
• Loud FastapiListingMigrationError instead of a generic traceback if you upgrade a custom
  subclass without migrating.

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What's Changed

• Make Filter/Sorter/Paginator/QueryStrategy backend-agnostic; add ClickHouse reference backend (0.4.0) by @danielhasan1 in #21

Full Changelog: v0.3.4...v0.4.0