fix: background jobs and lifespan must use BYPASSRLS sessions

All code that runs outside a request context (APScheduler jobs,
lifespan startup) has no app.current_account_id set, so the
app-role session returns 0 rows from every RLS-protected table.

Changed to _admin_session_factory (BYPASSRLS) in:
- knowledge_flywheel_scheduler.py — queries ai_sessions
- psa_retry_scheduler.py — queries psa_post_log
- retention_cleanup.py — queries assistant_chats
- scheduler.py (_fire_maintenance_schedule, _cleanup_expired_ai_conversations)
- main.py (archive_stale_ai_sessions, _process_notification_retries,
  load_all_schedules at startup)

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

backend/app/services/knowledge_flywheel_scheduler.py

@@ -10,7 +10,7 @@ import logging
 
 from sqlalchemy import select
 
-from app.core.database import async_session_maker
+from app.core.admin_database import _admin_session_factory as async_session_maker
 from app.models.ai_session import AISession
 from app.services.knowledge_flywheel import analyze_session
 

backend/app/services/psa_retry_scheduler.py

@@ -9,7 +9,7 @@ from datetime import datetime, timezone
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.core.database import async_session_maker
+from app.core.admin_database import _admin_session_factory as async_session_maker
 from app.models.psa_post_log import PsaPostLog
 from app.services.psa_documentation_service import retry_failed_push
 

backend/app/services/retention_cleanup.py

@@ -9,7 +9,7 @@ from datetime import datetime, timezone, timedelta
 
 from sqlalchemy import select, delete, func
 
-from app.core.database import async_session_maker
+from app.core.admin_database import _admin_session_factory as async_session_maker
 from app.models.account import Account
 from app.models.assistant_chat import AssistantChat