feat: auto-register Entity subclasses at class definition time

[deucalioncodes]

Summary

Add __init_subclass__ to Entity so that every subclass is automatically registered in Database._entity_types when the class is defined, not just when instances are created.

Problem

Entity types were only registered in _entity_types when:

1. An instance was created (Entity.__init__)
2. .instances() was called

After a canister upgrade, _entity_types started empty. Types that existed in storage but weren't touched during init were invisible — e.g. %db types wouldn't show them.

Every canister needed manual registration boilerplate at startup (see Realms' ~15-line loop).

Solution

• __init_subclass__ on Entity checks Database._instance directly (no auto-create)
• If DB exists → register immediately
• If not → append to Entity._deferred_types
• Database.init() flushes deferred types after creating the instance
• register_entity_type() is idempotent, so existing manual registration code is unaffected

Tests

8 new tests in test_auto_register.py:

• Type registered at definition time
• Multiple types registered
• Registration without creating instances
• Registration survives instance creation
• Namespaced entity registration
• Database.clear() preserves type registration
• _flush_deferred_types idempotency
• Deferred types flushed on Database.init()

Full suite: 122 passed (8 pre-existing failures in audit/database tests unchanged).

Closes #6