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

docs/architecture/stats-trends-data-flow.md

@@ -23,7 +23,9 @@ Trend charts now consume one chart-oriented backend payload from `/api/stats/tre
   - lookup rate trends
   - watch-time by day-of-week/hour
 - vocabulary-backed:
-  - new-words trend
+  - new-words trend reads permanent daily lexical rollups
+  - rollup rows count only vocabulary-visible tokens and normalize mixed legacy timestamp units
+  - a persisted rollup version invalidates stale materializations and triggers an atomic background rebuild
 
 ## Metric Semantics