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/models/__init__.py

@@ -9,10 +9,12 @@ from app.models.notification import Notification
 from app.models.auth import PasswordResetToken, ApiToken
 from app.models.company import Company
 from app.models.contact import Contact, CompanyContact
+from app.models.plugin import Plugin, PluginMigration
 
 __all__ = [
     "Tenant", "User", "UserTenant", "Role", "Session",
     "AuditLog", "DeletionLog", "Notification",
     "PasswordResetToken", "ApiToken", "Company",
     "Contact", "CompanyContact",
+    "Plugin", "PluginMigration",
 ]