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/storage/contracts.py

@@ -0,0 +1,55 @@
+from __future__ import annotations
+
+from datetime import datetime
+from typing import Protocol
+from uuid import uuid4
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+def _new_user_id() -> str:
+    return str(uuid4())
+
+
+class UserCreate(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
+    id: str = Field(default_factory=_new_user_id)
+    email: str
+    password_hash: str | None = None
+    system_role: str = "user"
+    oauth_provider: str | None = None
+    oauth_id: str | None = None
+    needs_setup: bool = False
+    token_version: int = 0
+
+
+class User(BaseModel):
+    model_config = ConfigDict(frozen=True)
+
+    id: str
+    email: str
+    password_hash: str | None
+    system_role: str
+    oauth_provider: str | None
+    oauth_id: str | None
+    needs_setup: bool
+    token_version: int
+    created_time: datetime
+    updated_time: datetime | None
+
+
+class UserRepositoryProtocol(Protocol):
+    async def create_user(self, data: UserCreate) -> User: ...
+
+    async def get_user_by_id(self, user_id: str) -> User | None: ...
+
+    async def get_user_by_email(self, email: str) -> User | None: ...
+
+    async def get_user_by_oauth(self, provider: str, oauth_id: str) -> User | None: ...
+
+    async def update_user(self, data: User) -> User: ...
+
+    async def count_users(self) -> int: ...
+
+    async def count_admin_users(self) -> int: ...