feat: consolidate config files to use CONFIG_DATADIR (fixes #3302)

[Noaman-Akhtar]

Description

• Created a centralized config_paths.py module with helper functions for config file paths
• Updated 6 existing files to use the new centralized path functions instead of hardcoded paths
• All config files now respect the CONFIG_DATADIR environment variable (already set in docker-compose.yml)
• Maintains backward compatibility when CONFIG_DATADIR is not set

This PR fixes #3302

Notes for Reviewers

• This builds on the work from PR Allow default config to be read from a file #3314 which introduced CONFIG_DATADIR
• Files changed:
  • augur/application/config_paths.py (NEW)
  • augur/application/db/engine.py
  • augur/application/cli/init.py
  • augur/application/cli/db.py
  • augur/tasks/git/util/facade_worker/facade_worker/config.py
  • augur/api/view/init.py
  • scripts/install/wizard.py
• When CONFIG_DATADIR is set (e.g., /config in Docker), config files are read from that directory
• When not set, defaults to current directory for backward compatibility

Signed commits

• Yes, I signed my commits.

[Noaman-Akhtar]

The remaining pylint errors (undefined-variable in db.py and unused imports in wizard.py) are pre-existing issues in the codebase, not introduced by this PR. Happy to fix them if you'd like, or they can be addressed in a separate PR.