chore: apply cargo fmt + clippy cleanup across crate

Silence forward-looking dead_code on unused DAO modules, annotate
individual placeholder items, rewrite tautological assert!(true/false)
in token tests as panic! arms, and pick up fmt drift.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/handlers.rs

@@ -503,7 +503,10 @@ pub async fn export_training_data_handler(
 
             HttpResponse::Ok()
                 .content_type("application/jsonl")
-                .insert_header(("Content-Disposition", "attachment; filename=\"training_data.jsonl\""))
+                .insert_header((
+                    "Content-Disposition",
+                    "attachment; filename=\"training_data.jsonl\"",
+                ))
                 .body(jsonl)
         }
         Err(e) => {