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/shared/ipc-channels.ts

@@ -257,6 +257,9 @@ export const ipcChannels = {
   activityHistoryReplace: 'activity-history:replace',
   activityHistoryUpdate: 'activity-history:update',
   activityHistoryClear: 'activity-history:clear',
+  activityHistoryPage: 'activity-history:page',
+  activityHistorySummary: 'activity-history:summary',
+  activityHistoryReconcile: 'activity-history:reconcile',
   tokenUsageSummary: 'usage:token-summary',
   tokenUsageChanged: 'usage:token-changed',
   artifactsList: 'artifacts:list',