Unify filter enums with generic Filterable trait

[jhodapp]

Summary

We have duplicate filter enum definitions across the entity_api and web layers. Each filter is defined twice with manual mapping in controllers. This should be unified using a trait-based pattern.

Current State

Three filter types are duplicated:

Filter	entity_api	web/params	Controller Mapping
RoleFilter	coaching_relationship.rs	coaching_relationship.rs:8	✅ Fixed via RoleFilterable trait
Scope	action.rs:239	action.rs:14	Manual match in action_controller.rs:64-67
AssigneeFilter	action.rs:227	action.rs:27	Manual match in action_controller.rs:70-74

Proposed Solution

Create a generic Filterable trait in entity_api/src/filter.rs:

// entity_api/src/filter.rs
pub trait Filterable {
    // Design TBD - could be:
    // fn to_condition(&self) -> Condition;
    // or a simpler boolean predicate pattern
}

Benefits

• Single source of truth for each filter (web layer only)
• No manual mapping in controllers
• Clear pattern for future filters
• Preserved dependency graph: web → domain → entity_api

Implementation Steps

1. Create entity_api/src/filter.rs with generic Filterable trait
2. Refactor RoleFilterable to use the new pattern (or keep as-is if different)
3. Create traits for Scope and AssigneeFilter
4. Remove duplicate enums from entity_api/src/action.rs
5. Update FindByUserParams to accept impl Trait instead of concrete types
6. Simplify action_controller.rs to pass web types directly
7. Update domain re-exports

Context

This was identified during PR #215 code review. The RoleFilter was fixed as a proof of concept, demonstrating the pattern works. The remaining filters should follow the same approach.

References

• PR Add user coaching role endpoints for What's Due dashboard #215: Add user coaching role endpoints for What's Due dashboard
• Commit cc16258: refactor: Unify RoleFilter using trait-based pattern