feat(threads): paginate full history via summaries endpoint (#1022)

* feat(threads): add paginated summaries API and load full history

* fix(threads): address summaries review feedback

- validate summaries sort params and log gateway failures
- page frontend thread summaries without stale query keys or silent truncation
- export router modules and tighten thread list typing

Refs: 2901804166, 2901804176, 2901804179, 2901804180, 2901804183, 2901804187, 2901804191

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/src/gateway/app.py

@@ -15,6 +15,7 @@ from src.gateway.routers import (
     models,
     skills,
     suggestions,
+    threads,
     uploads,
 )
 
@@ -139,6 +140,10 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
                 "name": "channels",
                 "description": "Manage IM channel integrations (Feishu, Slack, Telegram)",
             },
+            {
+                "name": "threads",
+                "description": "Thread summary and list endpoints for lightweight UI loading",
+            },
             {
                 "name": "health",
                 "description": "Health check and system status endpoints",
@@ -176,6 +181,9 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
     # Channels API is mounted at /api/channels
     app.include_router(channels.router)
 
+    # Threads API is mounted at /api/threads
+    app.include_router(threads.router)
+
     @app.get("/health", tags=["health"])
     async def health_check() -> dict:
         """Health check endpoint.