v1.23.2: Centralize credential resolution and normalize legacy environment aliases

Configuration & Authentication Refactoring (#210, #211)

• Centralized Credential Normalization:
  • Automated normalization of BASIC_AUTH_USERNAME and BASIC_AUTH_PASSWORD into canonical DASHBOARD_USERNAME and DASHBOARD_PASSWORD in Settings and web authentication resolvers.
• Deprecation Warnings:
  • Emits clear, actionable deprecation warnings when legacy BASIC_AUTH_* configuration variables are detected, alerting operators to transition.
• Zero Breaking Changes:
  • Full backward compatibility guaranteed for existing container deployments and Docker Compose stacks.
• Code Consolidation:
  • Deduplicated basic authentication verification logic into shared _verify_basic_credentials() helper in tunnel_sync.web.auth.
• Documentation & Verification:
  • Updated configuration documentation in docs/configuration.md and cf-tunnel-dashboard/DOCKERHUB.md.
  • Added unit test coverage for alias normalization and deprecation warnings in test_settings.py.
  • All 367 tests and 36 subtests passing cleanly across supported Python versions.
  • Reviews approved by @MustermenschAlex and @kuenstlichedummheit-sys.