Phase 4.6: Background tasks → TaskRegistry

[shikhalev]

Summary

• Migrate 6 background tasks from raw asyncio.create_task() to TaskRegistry (Phase 0 infrastructure)
• Extract task functions from orchestrator.py into domain modules
• Add graceful task cancellation on shutdown via task_registry.cancel_all()

Changes

4 new files (task functions extracted from orchestrator):

File	Tasks	Type
modules/core/maintenance.py	cleanup_expired_sessions (1h), periodic_vacuum (7d, initial_delay=24h)	periodic
modules/knowledge/tasks.py	build_wiki_embeddings, load_collection_indexes	one-shot
modules/kanban/tasks.py	sync_kanban_issues (15min, initial_delay=60s)	periodic
modules/ecommerce/tasks.py	woocommerce_daily_sync (daily 23:00 UTC)	one-shot with internal cron

orchestrator.py (1121 → 1030 lines, −91):

• Removed 6 inline task definitions + asyncio.create_task() calls
• Added TaskRegistry instance + register() + start_all() in startup
• Added cancel_all() in shutdown_event()
• Removed unused asyncio, datetime, timedelta imports

Design decisions

• WooCommerce sync registered as one-shot (manages its own cron-style sleep loop internally) since TaskRegistry doesn't support cron scheduling
• Wiki RAG tasks use functools.partial(fn, wiki_rag) to pass the service instance, since TaskRegistry.register() expects a zero-arg callable
• Core maintenance in modules/core/maintenance.py (not tasks.py) to avoid name collision with TaskRegistry infrastructure in modules/core/tasks.py
• Lazy imports preserved in task functions to avoid circular import risk

Test plan

• ruff check — clean
• pytest tests/unit/ — 65 tests pass (including 8 TaskRegistry tests)
• Docker deploy: verify tasks start correctly in logs

Closes #551

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