fix(explain): address review findings — N+1 queries, duplicate decisions, silent errors

1. fetch_open_threads: replace N+1 loop (2 queries per thread) with a
   single query using correlated subqueries for note_count and started_by.
2. extract_key_decisions: track consumed notes so the same note is not
   matched to multiple events, preventing duplicate decision entries.
3. build_timeline_excerpt_from_pipeline: log tracing::warn on seed/collect
   failures instead of silently returning empty timeline.

src/timeline/seed.rs

@@ -411,7 +411,9 @@ fn round_robin_select_by_discussion(
         let mut made_progress = false;
 
         for (disc_idx, &discussion_id) in discussion_order.iter().enumerate() {
-            let notes = by_discussion.get(&discussion_id).unwrap();
+            let notes = by_discussion
+                .get(&discussion_id)
+                .expect("key present: inserted into by_discussion via discussion_order");
             let note_idx = indices[disc_idx];
 
             if note_idx < notes.len() {