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/plugin.toml

@@ -0,0 +1,18 @@
+[plugin]
+name = "auth"
+summary = "Cookie-based authentication and authorization"
+version = "0.1.0"
+description = "Owns DeerFlow authentication, authorization adapters, and auth storage definitions while reusing shared persistence infrastructure."
+author = "DeerFlow"
+tags = ["auth", "gateway", "session"]
+
+[capabilities]
+router = true
+middleware = true
+dependencies = true
+langgraph_adapter = true
+storage = true
+
+[storage]
+mode = "shared_infrastructure"
+notes = "This plugin owns its storage definitions and repositories but uses the application's shared engine and session factory."