fix(stats): version lexical rollups and preserve backfill writes

- Persist vocabulary_visible on imm_words and filter rollups/triggers by it, matching the totals' exclusion rules
- Normalize legacy second/millisecond timestamps when bucketing rollup rows into local days
- Replace the boolean rollup-ready flag with a version key so schema changes trigger an atomic rebuild
- Preserve queued playback writes during rollup backfill instead of dropping them under queue pressure
- Time out the lexical rollup worker instead of hanging forever on no response
- Key concurrent vocabulary summary requests by their known-words snapshot so they no longer share stale state
- Bound stats dashboard retries for unfinished vocabulary loads before showing the inline Retry control

src/core/services/stats-sync/merge-catalog.ts

@@ -89,6 +89,7 @@ const WORD_COPY_COLUMNS = [
   'last_seen',
   'frequency',
   'frequency_rank',
+  'vocabulary_visible',
 ] as const;
 
 export function mergeAnime(