T03: plugin system framework + lifecycle + migrations + event bus + DI

- Plugin registry with discover/install/activate/deactivate/uninstall lifecycle
- PluginManifest Pydantic v2 schema (name, version, dependencies, routes, events, migrations)
- BasePlugin abstract class with lifecycle hooks (on_install/activate/deactivate/uninstall)
- Migration runner with tenant_id validator (rejects tables without tenant_id)
- Event bus integration: register/unregister listeners on activate/deactivate
- Service container DI: plugins receive db, cache, event_bus, storage, notifications
- Idempotent operations (activate active=200, deactivate inactive=200)
- UI registry for frontend component registration
- 47 new tests (14 ACs + 33 unit tests), 103 total tests pass
- Migration 0003: plugins + plugin_migrations tables
- Coverage: 85.92% for plugin modules

app/plugins/__init__.py

@@ -0,0 +1,8 @@
+"""Plugin system framework for LeoCRM."""
+
+from app.plugins.manifest import PluginManifest
+from app.plugins.base import BasePlugin
+from app.plugins.registry import get_registry
+from app.plugins.migration_runner import MigrationRunner
+
+__all__ = ["PluginManifest", "BasePlugin", "get_registry", "MigrationRunner"]