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/builtins/migrations/0001_bad_migration.sql

@@ -0,0 +1,7 @@
+-- Bad migration: creates table WITHOUT tenant_id (should be rejected by validator)
+CREATE TABLE IF NOT EXISTS plugin_bad_table (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    title VARCHAR(200) NOT NULL,
+    content TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);