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/schemas/plugin.py

@@ -0,0 +1,61 @@
+"""Pydantic schemas for plugin API responses."""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field
+
+
+class PluginInfo(BaseModel):
+    """Plugin status information returned in list endpoint."""
+
+    name: str
+    display_name: str
+    version: str
+    status: str = Field(description="discovered | installed | active | inactive")
+    installed: bool = False
+    active: bool = False
+    description: str = ""
+    dependencies: list[str] = Field(default_factory=list)
+    events: list[str] = Field(default_factory=list)
+    migrations: list[str] = Field(default_factory=list)
+    permissions: list[str] = Field(default_factory=list)
+
+
+class PluginListResponse(BaseModel):
+    """Response for GET /api/v1/plugins."""
+
+    plugins: list[PluginInfo]
+    total: int
+
+
+class PluginActionResponse(BaseModel):
+    """Response for install/activate/deactivate/uninstall actions."""
+
+    name: str
+    display_name: str
+    version: str
+    status: str
+    installed: bool = False
+    active: bool = False
+    dropped_tables: list[str] = Field(default_factory=list, description="Tables dropped (uninstall with remove_data=true)")
+    message: str = ""
+
+
+class PluginUninstallResponse(PluginActionResponse):
+    """Response for DELETE /api/v1/plugins/{name}."""
+
+    pass
+
+
+class ManifestFieldDoc(BaseModel):
+    """Documentation for a single manifest field."""
+    type: str
+    required: str
+    description: str
+
+
+class ManifestSchemaResponse(BaseModel):
+    """Response for GET /api/v1/plugins/manifest."""
+
+    fields: dict[str, ManifestFieldDoc]
+    example: dict