Refactor user and auth logic into dedicated service layer

[Copilot]

Extract authentication and user management logic from controllers into dedicated service classes following existing service patterns (ai_agents_service, routing_service).

Changes

New Services

• app/services/auth_service.py - Password hashing/verification, user authentication, token generation, JWT validation
• app/services/user_service.py - User CRUD operations, validation (username/password length, uniqueness checks)

Refactored

• app/core/security.py - Reduced to JWT token creation and OAuth2 scheme definition only
• app/api/v1/endpoints/auth.py - Removed direct DB queries and validation; delegates to services
• app/api/v1/endpoints/users.py - Uses auth_service.get_current_user instead of core.security

Before/After

# Before: Controller handled everything
@router.post("/register")
async def register_user(user_in: UserCreate, db: Session = Depends(get_db)):
    user = db.query(User).filter(User.username == user_in.username).first()
    if user:
        raise HTTPException(400, "User exists")
    # ... validation, hashing, token generation inline

# After: Delegated to services  
@router.post("/register")
async def register_user(user_in: UserCreate, db: Session = Depends(get_db)):
    hashed_password = auth_service.get_password_hash(user_in.password)
    user = user_service.create_user(db, user_in, hashed_password)
    return {"access_token": auth_service.generate_access_token(user.id), ...}

All tests updated to import from service layer. No breaking changes to API contracts.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Refactor the user service</issue_title>
<issue_description>- Separate the user service from the controller

• Seperate the auth service from the controller
• Move core.security to a service concerning users authentificatoin
• (Potentially) adjust tests for change in structure</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Refactor the user service #35

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