Refactor filtering, ordering and pagination

[Flo0807]

Filters

Core

• Created Backpex.FilterValidation for changeset-based validation
  • build_changeset/3: builds schemaless changeset from URL params
  • valid_values/1: extracts only validated filter values
  • build_types/2: builds type map from filter configurations

New Filter Callbacks

• Added type/1: specifies Ecto type for casting URL params
• Added changeset/3: allows custom validation logic per filter
• Added validate/2: public API for testing validation

Existing Filter Updates

• Boolean Filter: validates selected keys exist in options/1
• Select Filter: validates selected value exists in options/1
• MultiSelect Filter: validates all selected values exist in options/1
• Range Filter: validates date/number formats and that start <= end

LiveResource

• Updated index.ex to use FilterValidation.build_changeset/3
• Added @filter_form assign (Phoenix form from changeset)
• Added @filter_values assign (only valid, casted values)
• Invalid filters are not applied to queries: users see unfiltered results with inline errors

Ecto Adapter

• Changed apply_filters/4 to apply_filters/3 (removed empty_filter_key parameter)
• Filters now receive validated, casted values

Ordering & Pagination

• Created lib/backpex/query_options_validation.ex for validating URL parameters
  • Uses Ecto changesets for integer validation (page, per_page)
  • Safe atom conversion for order_by/order_direction
  • Invalid values fall back to defaults instead of crashing
• Invalid values silently fall back to defaults