Add auto memory distillation scheduler (APScheduler)

- scheduler.py: AsyncIOScheduler with three cron jobs
    short  daily     03:00 (no LLM, always fast)
    mid    weekly    Sun 03:30 (LLM)
    long   monthly   1st 04:00 (LLM — off by default)
- config.py: AUTO_DISTILL, AUTO_DISTILL_SHORT/MID/LONG .env flags
- main.py: start/stop scheduler in FastAPI lifespan
- routers/distill.py: GET /distill/status — next run times + config
- requirements.txt: apscheduler>=3.10
- HELP.md: updated planned items, added /distill/status to API table

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

cortex/main.py

@@ -13,7 +13,10 @@ from routers import chat, google_chat, nextcloud_talk, files, distill
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    import scheduler
+    scheduler.start()
     yield
+    scheduler.stop()
     from llm_client import cleanup
     await cleanup()