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/app/handlers.rs

@@ -735,7 +735,7 @@ async fn handle_init(
         }
 
         let project_paths: Vec<String> = projects_flag
-            .unwrap()
+            .expect("validated: checked for None at lines 714-721")
             .split(',')
             .map(|p| p.trim().to_string())
             .filter(|p| !p.is_empty())
@@ -743,8 +743,10 @@ async fn handle_init(
 
         let result = run_init(
             InitInputs {
-                gitlab_url: gitlab_url_flag.unwrap(),
-                token_env_var: token_env_var_flag.unwrap(),
+                gitlab_url: gitlab_url_flag
+                    .expect("validated: checked for None at lines 714-721"),
+                token_env_var: token_env_var_flag
+                    .expect("validated: checked for None at lines 714-721"),
                 project_paths,
                 default_project: default_project_flag.clone(),
             },