Expose ComfyUI run time on Enhance jobs as run_ms

Parse execution_start vs the end event from /history, store it when the
poller moves a job to ready/failed (never overwritten with null), and
return it as "run_ms" on every job endpoint.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/database/schema.rs

@@ -60,6 +60,7 @@ diesel::table! {
         discarded -> Integer,
         created_at -> BigInt,
         updated_at -> BigInt,
+        run_ms -> Nullable<BigInt>,
     }
 }