who.rs: Add INDEXED BY idx_notes_diffnote_path_created to all DiffNote
query paths (expert, expert_details, reviews, path probes, suffix_probe).
SQLite planner was choosing idx_notes_system (106K rows, 38%) over the
partial index (26K rows, 9.3%) when LIKE predicates are present.
Measured: expert 1561ms->59ms (26x), reviews ~1200ms->16ms (75x).
stats.rs: Replace 12+ sequential COUNT(*) queries with conditional
aggregates (SUM(CASE WHEN...)) and use FTS5 shadow table
(documents_fts_docsize) instead of virtual table for counting.
Measured: warm 109ms->65ms (1.68x).
740607e06d