fix(stats): preserve session rollups across schema upgrades

- Stop deleting imm_daily_rollups/imm_monthly_rollups on unrelated schema version bumps; their source session/telemetry rows may already be pruned, so deleted buckets could not be rebuilt
- Run startup session-rollup maintenance before the lexical rollup backfill takes the write lock, so recovery no longer races playback writes
- Update the vocabulary summary totals changelog fragment to reflect that watch-time, activity, efficiency, and library charts are no longer cleared during rebuilds

src/core/services/immersion-tracker-service.ts

@@ -591,8 +591,8 @@ export class ImmersionTrackerService {
       }
     }
     this.preparedStatements = createTrackerPreparedStatements(this.db);
-    if (!areLexicalDailyRollupsReady(this.db)) this.startLexicalRollupBackfill();
     this.scheduleMaintenance();
+    if (!areLexicalDailyRollupsReady(this.db)) this.startLexicalRollupBackfill();
     this.scheduleFlush();
   }