feat(dimentorin): mentor stats endpoint public - total sessions, unique mentees, avg rating

- GET /mentors/{id}/stats (public, no auth)
- resolve mentor profile id -> user id in get_mentor_sessions (FK uses app_users.id)

imphnen-dimentorin/src/sessions/infrastructure/http/handlers/mod.rs

@@ -5,5 +5,5 @@ pub use mutation_handlers::{
 	post_book_session, post_submit_feedback, put_update_session_status,
 };
 pub use query_handlers::{
-	get_mentor_availability, get_mentor_sessions, get_my_sessions,
+	get_mentor_availability, get_mentor_sessions, get_mentor_stats, get_my_sessions,
 };