Refactor auth service to use FastAPI dependency injection chain

[lbedner]

Problem

Auth routes manually instantiate UserService(db) in every endpoint handler:

# app/components/backend/api/auth/router.py — repeated in every route
async def register(user_data: UserCreate, db: AsyncSession = Depends(get_async_db)):
    user_service = UserService(db)
    ...

This creates boilerplate and bypasses FastAPI's dependency injection system. The service is stateful (holds a db session) but isn't wired as a proper dependency.

Proposed Change

Add a get_user_service dependency that composes get_async_db, so routes receive the service directly:

# app/components/backend/api/deps.py (or auth-specific deps)
async def get_user_service(
    db: AsyncSession = Depends(get_async_db),
) -> UserService:
    return UserService(db)

Then routes simplify to:

@router.post("/register", response_model=UserResponse)
async def register(
    user_data: UserCreate,
    user_service: UserService = Depends(get_user_service),
):
    existing_user = await user_service.get_user_by_email(user_data.email)
    ...

Similarly, get_current_user_from_token in auth_service.py manually creates a UserService(db) — this should also use the injected service.

Files to Modify

• app/components/backend/api/deps.py — add get_user_service dependency
• app/components/backend/api/auth/router.py — replace manual UserService(db) with Depends(get_user_service) in all 8 route handlers
• app/services/auth/auth_service.py — update get_current_user_from_token to accept UserService instead of raw db

Benefits

• Eliminates repeated UserService(db) boilerplate across 8 endpoints
• Proper FastAPI DI — services are composable and testable via app.dependency_overrides
• Easier to mock in tests (override one dependency vs patching constructors)