feat(backend): FastAPI + libSQL + auth register/login/me + tests (TDD)

- FastAPI app with lifespan, CORS, health endpoint
- SQLAlchemy 2.0 async with aiosqlite, Base/UUIDMixin/TenantMixin/TimestampMixin
- Tenant and User models with multi-tenant isolation
- Auth: register (creates tenant+user), login, /me endpoint
- JWT HS256 tokens, bcrypt password hashing
- Alembic async setup with initial migration
- 6 passing tests (register, login, wrong password, me, no token, health)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/config.py

@@ -0,0 +1,15 @@
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(env_file=".env")
+
+    SECRET_KEY: str = "dev-secret-change-me"
+    DATABASE_URL: str = "sqlite+aiosqlite:///./data/roaauto.db"
+    ACCESS_TOKEN_EXPIRE_DAYS: int = 30
+    TRIAL_DAYS: int = 30
+    SMSAPI_TOKEN: str = ""
+    CORS_ORIGINS: str = "http://localhost:5173"
+
+
+settings = Settings()