Django 5.2 prep: STORAGES dict + drop USE_L10N (#1081)

[rdhyee]

First concrete step toward the Django 4.2 → 5.2 LTS upgrade (#1081). Forward-compatible settings changes that behave identically on the current 4.2 runtime and remove two 5.x landmines.

Changes

• STORAGES dict replaces DEFAULT_FILE_STORAGE (removed in Django 5.1). STORAGES is supported since 4.2, so this is a no-op on the live runtime; the S3 (django-storages) vs FileSystemStorage selection is preserved.
• Drop USE_L10N (removed in Django 5.0; localized formatting is always on).

Why it's small

The other landmines from PLAN_django52_upgrade.md are already clean here: no django.utils.timezone.utc, no index_together, no removed util imports (force_text/ugettext/…). The real remaining work is the dependency audit (django-selectable, django-registration, ckeditor5) — separate PRs.

⚠️ Validation needed before merge

The STORAGES change touches the prod S3 media path. Confirm media upload + serve on test.unglue.it before merging (per the plan's "test, don't just rename").

No code reads settings.DEFAULT_FILE_STORAGE directly (checked), and no STATICFILES_STORAGE was set, so static handling is unchanged.

[rdhyee]

🤖 Codex review: LGTM (no changes). Confirmed: STORAGES is behavior-identical on Django 4.2 (same S3-vs-FileSystem selection, just relocated); explicit staticfiles entry matches Django's default; DEFAULT_FILE_STORAGE removed so no ImproperlyConfigured risk; USE_L10N removal is a no-op (default already True). Reminder: still validate S3 media upload/serve on test.unglue.it before merge.

[rdhyee]

⚠️ Deploy coordination required (found by Codex during the #1203 spike review): the Ansible-rendered prod.py (provisioning prod.py.j2:175) still sets DEFAULT_FILE_STORAGE after common.py — on Django 4.2 that combination raises ImproperlyConfigured (reproduced locally) → site-wide 500 at boot if this PR deploys alone. Merge+deploy EbookFoundation/regluit-provisioning#61 in the same window (config render before/with app restart).