Refactor auth and setup services

- Updated auth_service.py to improve authentication logic
- Modified setup_service.py for better configuration handling
- Added comprehensive tests for setup_service
- Updated documentation in Tasks.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/services/setup_service.py

@@ -208,14 +208,6 @@ async def run_setup(
     log.info("bangui_setup_completed")
 
 
-async def get_password_hash(
-    db: aiosqlite.Connection,
-    settings_repo: SettingsRepository = default_settings_repo,
-) -> str | None:
-    """Return the stored bcrypt password hash, or ``None`` if not set."""
-    return await settings_repo.get_setting(db, _KEY_PASSWORD_HASH)
-
-
 async def get_runtime_database_path(
     db: aiosqlite.Connection,
     settings_repo: SettingsRepository = default_settings_repo,