perf(activity): page, summarize and reconcile activity history in Main by index

Schema 57 rebuilds activity_history_records with order_seq (newer = larger;
a move to the front assigns a new maximum) and stored generated columns
(record_id, occurrence, status, status_group, kind, request_id,
conversation_id, project_id, created_at) with indexes. The order is migrated
from record_order_json exactly; legacy duplicates keep their occurrence and
only the first is shown. record_order_json is no longer written.

ActivityHistoryRepository owns the SQL: page({before, limit <= 500, filter,
projectId, conversationId}) newest first by index, summary() with the counts
and conversation titles/status the Activity page derived from the full list,
reconcile() joined with tasks with a renderer-localized detail line, update()
applied directly in SQL without reading the history, replace() for the
legacy migration, clear(). New activity-history:page/summary/reconcile IPC
with zod validation; page and summary run on the readonly worker.
perf:main measures 24,000 records: startup load 106 ms on Main (14 MB) ->
first page + summary 13.5 ms on the worker (0.6 ms loop block, 114 KB).

src/main/readonly-query-worker.ts

@@ -36,7 +36,9 @@ if (kind === 'knowledge') {
     listConversationSummaries: ([detailIds]) => database.readSnapshot(() =>
       database.listConversationSummaries(detailIds as string[])),
     getConversation: ([conversationId]) => database.readSnapshot(() =>
-      database.getConversation(conversationId as string))
+      database.getConversation(conversationId as string)),
+    activityHistoryPage: ([input]) => database.getActivityHistoryPage(input),
+    activityHistorySummary: ([input]) => database.getActivityHistorySummary(input)
   }
 }