Add comprehensive implementation plan for generic saved filters feature:
- Generic /api/saved-filters endpoint with resource_type field
- Service layer architecture with business logic
- JSONB storage for flexible filter schemas
- Integration test patterns
- Support for both JSON (API) and HTML (HTMX) responses
- Database schema with auto-updating updated_at trigger
This plan follows PROJECT_GUIDELINES.md and matches existing codebase patterns
(service layer, handler constructors, error handling, testing patterns).
Related to bookshelf page save filter functionality.
