ISSUE-277 Artifact — shared i18n registry parity report (Django ↔ Angular MVP)

Issue: #277 ([READY][#175][P3] Shared i18n registry artifact: backend/frontend keyspace parity report)

Artifact metadata

Single canonical artifact per issue: issue-bundne rapporter navngives docs/ISSUE-<nr>-<slug>-ARTIFACT.md.
Stable artifact identity: artifact_id ændres ikke ved tekstlige opdateringer i samme rapporttype; det er den faste reference i review/ops.
Explicit artifact versioning: artifact_version bumpes, når rapportlogik eller scope ændres, så drift/review kan se forskel på format- vs. dataændringer.
Shared namespace first: keys refereres med fulde navnerum (frontend.ui.*, frontend.errors.*, backend.error_codes.*, backend.errors.*) i stedet for lokale aliases i artefakter.
Source-of-truth before consumers: rapporten afledes fra shared/i18n/lobby.json; Django/Angular beskrives som consumers af samme registry og ikke som parallelle kontrakter.

Frontend UI gameplay keys checked: 16 → OK
Frontend error keys checked: 7 → OK
Backend gameplay/error codes checked: 9 → OK
Distinct frontend error keys reached from backend MVP flow: 6 (join_failed, nickname_invalid, nickname_taken, session_code_required, session_not_found, start_round_failed)

Status: Shared locale matrix is aligned (en, da) and backend→frontend error handling is contract-complete for MVP-critical flow.

Backend code (`backend.error_codes.*`)	Django message key (`backend.errors.*`)	Angular key (`frontend.errors.*`)	Locales `en/da`	Parity note
`session_code_required`	`session_code_required`	`session_code_required`	`OK`	1:1
`nickname_invalid`	`nickname_invalid`	`nickname_invalid`	`OK`	1:1
`session_not_found`	`session_not_found`	`session_not_found`	`OK`	1:1
`session_not_joinable`	`session_not_joinable`	`join_failed`	`OK`	mapped alias
`nickname_taken`	`nickname_taken`	`nickname_taken`	`OK`	1:1
`category_slug_required`	`category_slug_required`	`start_round_failed`	`OK`	many:1 collapse
`category_not_found`	`category_not_found`	`start_round_failed`	`OK`	many:1 collapse
`round_start_invalid_phase`	`round_start_invalid_phase`	`start_round_failed`	`OK`	many:1 collapse
`round_already_configured`	`round_already_configured`	`start_round_failed`	`OK`	many:1 collapse

Django consumes backend codes/messages directly from shared/i18n/lobby.json via lobby/i18n.py.
Angular consumes the same registry via frontend/shared/i18n/lobby-loader.ts and runtime helpers in frontend/angular/src/app/lobby-i18n.ts.
Parity in MVP is therefore strongest on the shared error contract and locale matrix; gameplay UI labels are frontend-owned but still live in the same registry.

Error granularity collapse remains intentional: backend codes category_slug_required, category_not_found, round_start_invalid_phase, round_already_configured all map to frontend.errors.start_round_failed. Follow-up only if product wants case-specific Angular copy instead of one shared host failure message.
Frontend-only fallback copy is not mirrored in Django: frontend.errors.unknown and frontend.errors.session_fetch_failed are Angular-side resilience keys, not backend contract keys. Follow-up if API responses should expose stable backend equivalents for these states.
Gameplay UI labels are registry-shared but not backend-rendered: frontend.ui.host.*, frontend.ui.player.*, and frontend.ui.common.* are available in the shared artifact, but Django currently consumes only the backend error slice. Follow-up only if server-rendered views must guarantee the same UI label surface as Angular.

python3 scripts/check_i18n_drift.py
python3 scripts/report_i18n_parity.py
python3 scripts/check_i18n_parity_artifact.py