Convert `getattr(settings, "FOO")` to `settings.FOO`

Rationale

• Documentation
  • With getattr, it is possible to ad-hoc check settings which were never defined in (lms,cms,openedx)/envs/common.py, adding hidden config knobs to the platform.
  • With .settings, there must be a common.py definition, which helps operators reason about configuration and gives us a place to document settings.
• Robustness
  • With getattr, a misspelling of FOO will silently fall to the default.
  • With settings. a misspelling of FOO will raise an AttributeError which we can find and fix
• Analysis
  • getattr invocations are often skipped by mypy, pylint, ruff, etc due to their dynamic nature
  • Regular calls to settings. can be statically analyzed for type correctness etc
• Defaults
  • With getattr, you can have conflicting defaults defined in 2+ different places:
    • the definition, if there is one (FOO = "<default1>")
    • each call site getattr(settings, "FOO", <default2>), getattr(settings, "FOO", <default3>), ...
  • With settings., there's exactly one default: the definition (FOO = "<default1>")

Tasks

Roughly:

1. Make a quick ADR which we can use as guidance for devs who reference settings in the future
2. Catalog all invocations of getattr(settings, ...) and make a best guess as to which services use them (LMS, CMS, both -- will be hard to determine this with 100% confidence)
3. Add the necessary definitions to (openedx,lms,cms)/envs/common.py.
   • Define it with a default which does not change the behavior of the base system
   • This will involve, in some cases, moving definitions up from lms/envs/common.py into openedx/envs/common.py
4. Convert the getattr(settings, "FOO", <default>) call site into settings.FOO.
5. Tests will fail if a setting was defined in the wrong place. Fix and repeat.

It might make sense to do 3-5 in batches

Start by cataloging getattr(settings, ...) invocations across the repository and review the existing definitions in openedx, lms, and cms/envs/common.py. Draft the ADR, identify which services use each setting, and run the relevant tests after converting settings references in batches. Done means required settings have behavior-preserving defaults in the appropriate common.py files and the test suite passes.