feat: add APScheduler task to auto-archive stale AI chat sessions

Archives AI chat sessions with no activity for 30 days, runs daily at 3 AM.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -22,6 +22,27 @@ setup_logging()
 logger = logging.getLogger(__name__)
 
 
+async def archive_stale_ai_sessions():
+    """Archive AI chat sessions with no activity for 30 days."""
+    from app.models.ai_chat_session import AIChatSession
+    from sqlalchemy import update
+    from datetime import datetime, timezone, timedelta
+
+    cutoff = datetime.now(timezone.utc) - timedelta(days=30)
+    async with async_session_maker() as db:
+        result = await db.execute(
+            update(AIChatSession)
+            .where(
+                AIChatSession.updated_at < cutoff,
+                AIChatSession.archived_at.is_(None),
+                AIChatSession.status != "abandoned",
+            )
+            .values(archived_at=datetime.now(timezone.utc))
+        )
+        await db.commit()
+        logger.info(f"[archive] Archived {result.rowcount} stale AI chat sessions")
+
+
 def _configure_seed_module(mod: object, api_url: str, email: str, password: str) -> None:
     """Set globals on a seed script module."""
     mod.API_BASE_URL = api_url  # type: ignore[attr-defined]
@@ -132,6 +153,15 @@ async def lifespan(app: FastAPI):
         replace_existing=True,
     )
 
+    # Auto-archive stale AI chat sessions (daily at 3 AM)
+    scheduler.add_job(
+        archive_stale_ai_sessions,
+        "cron",
+        hour=3,
+        id="archive_stale_ai_sessions",
+        replace_existing=True,
+    )
+
     # Auto-seed trees in background on PR environments
     seed_task = None
     if settings.SEED_ON_DEPLOY: