refactor: decouple DefaultQueryProvider, search_utils, and Filter classes from SQLAlchemy

[borhanst]

Summary

• Replace direct SQLAlchemy imports in DefaultQueryProvider and apply_search_filter() with QueryBackend and IntrospectionBackend from app.state
• Add ilike(), or_(), options() to QueryBackend protocol and SqlAlchemyQueryAdapter
• Add introspection methods to IntrospectionBackend protocol and SqlAlchemyIntrospectionAdapter
• Update Filter ABC and all 7 subclasses to accept QueryBackend and model separately
• Remove all direct SQLAlchemy imports from filters/base.py
• Update FilterRegistry.auto_generate() to use IntrospectionBackend
• All 580 tests pass, linter clean

Closes #26
Closes #27