feat(sync): use incremental session index after pull sync

After pull-sync imports new messages, generate session index incrementally
instead of full regeneration. This preserves existing sessions and their
summaries, only processing newly imported (unindexed) messages.

Adds generateIncremental method across the full stack:
- core: already had generateIncrementalSessionIndex
- node-runtime: expose generateIncrementalIndex
- CLI: add POST /_web/sessions/:id/generate-incremental-index route
- Electron: add session:generateIncremental IPC handler
- Frontend: add generateIncremental to SessionIndexAdapter interface

packages/node-runtime/src/services/index.ts

@@ -24,7 +24,14 @@ export {
 export type { MembersPaginatedDTO } from './member-service'
 
 // Session index service
-export { generateIndex, clearIndex, getFtsStatus, searchFts, rebuildFts } from './session-index-service'
+export {
+  generateIndex,
+  generateIncrementalIndex,
+  clearIndex,
+  getFtsStatus,
+  searchFts,
+  rebuildFts,
+} from './session-index-service'
 
 // Summary service
 export { generateSummary, generateAllSummaries } from './summary-service'

packages/node-runtime/src/services/session-index-service.ts

@@ -6,6 +6,7 @@
 
 import {
   generateSessionIndex as coreGenerateSessionIndex,
+  generateIncrementalSessionIndex as coreGenerateIncrementalSessionIndex,
   clearSessionIndex as coreClearSessionIndex,
 } from '@openchatlab/core'
 import { hasFtsTable, searchByFts, rebuildFtsIndex } from '../fts'
@@ -16,6 +17,15 @@ export function generateIndex(adapter: SessionRuntimeAdapter, sessionId: string,
   return coreGenerateSessionIndex(db, gapThreshold)
 }
 
+export function generateIncrementalIndex(
+  adapter: SessionRuntimeAdapter,
+  sessionId: string,
+  gapThreshold: number = 1800
+): number {
+  const db = adapter.ensureWritable(sessionId)
+  return coreGenerateIncrementalSessionIndex(db, gapThreshold)
+}
+
 export function clearIndex(adapter: SessionRuntimeAdapter, sessionId: string): void {
   const db = adapter.ensureWritable(sessionId)
   coreClearSessionIndex(db)