feat(app): add plugin system with auth plugin and static assets

Add new application structure:
- app/main.py - application entry point
- app/plugins/ - plugin system with auth plugin:
  - api/ - REST API endpoints and schemas
  - authorization/ - auth policies, providers, hooks
  - domain/ - business logic (service, models, jwt, password)
  - injection/ - route injection and guards
  - ops/ - operational utilities
  - runtime/ - runtime configuration
  - security/ - middleware, CSRF, dependencies
  - storage/ - user repositories and models
- app/static/ - static assets (scalar.js for API docs)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/plugins/auth/__init__.py

@@ -0,0 +1,14 @@
+"""Auth plugin package.
+
+Level 2 plugin goal:
+- Own auth domain logic
+- Own auth adapters (router, dependencies, middleware, LangGraph adapter)
+- Own auth storage definitions
+- Reuse the application's shared persistence/session infrastructure
+"""
+
+from app.plugins.auth.authorization.hooks import build_authz_hooks
+
+__all__ = [
+    "build_authz_hooks",
+]