feat: add FrontendSiteConfigView for frontend-base site configuration

[arbrandes]

Description

• Adds a new FrontendSiteConfigView endpoint at /api/mfe_config/v1/frontend_site that translates the existing flat MFE_CONFIG / MFE_CONFIG_OVERRIDES Django settings into the camelCase structure expected by frontend-base's SiteConfig.
• Refactors the existing MFEConfigView to use shared helper functions (get_legacy_config, get_mfe_config, get_mfe_config_overrides), reducing duplication.
• Keys matching RequiredSiteConfig / OptionalSiteConfig are promoted to the top level under their camelCase names; remaining keys go into commonAppConfig (pending feat: support commonAppConfig site config option frontend-base#179); each MFE_CONFIG_OVERRIDES entry becomes an element of the apps array.

Work log (Claude Code sessions)

Session 1: Initial implementation

Started from a scaffold FrontendSiteConfigView with a docstring describing the desired behavior. Built the implementation iteratively:

1. Core translation logic — Implemented SITE_CONFIG_TRANSLATION_MAP mapping all known RequiredSiteConfig and OptionalSiteConfig fields from frontend-base's types.ts. Keys in MFE_CONFIG matching the map are promoted to camelCase top-level fields; all others become app configuration.
2. Removed "custom" section — Initial implementation put unmapped keys in a custom dict; this was removed in favor of including them directly in app config.
3. Per-app config from overrides — Each MFE_CONFIG_OVERRIDES entry becomes an element of the apps array with its overrides merged on top of the shared base config.
4. Helper extraction — Refactored get_legacy_config(), get_mfe_config(), and get_mfe_config_overrides() into shared module-level functions, eliminating duplication between MFEConfigView and the new view. Removed the intermediate get_merged_mfe_config() function in favor of inline composition.
5. Legacy config optimization — Moved get_legacy_config() directly into app_base_config since none of its keys map to frontend-base types, avoiding a split-then-recombine step.
6. Site-level key stripping — Added filtering to remove SITE_CONFIG_TRANSLATION_MAP keys from per-app override configs so they don't leak into app configuration.
7. mfe_name_to_app_id helper — Converts kebab-case MFE names to reverse-domain appIds (e.g. learner-dashboard → org.openedx.frontend.app.learnerDashboard).
8. Tests — Wrote tests for each of the above behaviors, updating them as the implementation evolved.

Session 2: Code review and commonAppConfig refactor

A code review identified that per-app configs were duplicating all shared keys (legacy + unmapped MFE_CONFIG) into every app entry. This was both wasteful and divergent from frontend-base's design where shared config should be separate.

• Discussed whether the lost-keys problem (site-level keys from overrides being silently dropped) could be solved without a pre-defined app list — concluded it cannot, since the endpoint doesn't know which apps exist beyond what's in overrides.
• Introduced commonAppConfig as a top-level field containing legacy config + unmapped MFE_CONFIG keys, shared across all apps. Per-app entries in apps now carry only their own overrides, since frontend-base merges commonAppConfig into each app's config client-side.

Session 3: Final review

A final review confirmed the implementation is correct and identified remaining items for consideration:

• Per-app site-level keys in overrides (e.g. per-app BASE_URL) are silently discarded — by design, since frontend-base treats these as site-wide, but worth documenting for operators.
• siteId (a RequiredSiteConfig field in frontend-base) has no legacy Django equivalent and is not emitted.
• The URL path uses a leading slash (/frontend_site) as a workaround for the parent URL lacking a trailing slash.

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