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.
Add pre-flight FTS count check before expensive bm25-ranked search.
Queries matching >10,000 documents are rejected instantly with a
suggestion to use a more specific query or --since filter.
Prevents multi-minute CPU spin on queries like 'merge request' that
match most of the corpus (106K/178K documents).
